EP 3 800 000 A1 (continued) LONGITUDINAL INTERVAL SIZE  (mm) RATIO OF LONGITUDINAL INTERVAL SIZE TO  LONGEST DIAMETER ADHESION  STRENGTH (N) 0.6 600% 868.76 0.65 650% 868.54 0.7 700% 821.88 0.75 750% 800.4 0.8 800% 788.66 0.85 850% 790.56 0.9 900% 784.22 0.95 950% 786.55 1 1000% 782.46 [0038] When the surface of the CFRP member (the member S) had not been processed with roughening treatment, the adhesion strength was measured as 785.2N. As shown in the experimental results, when the longitudinal interval size was too short, the adhesion strength was weaker than when the CFRP member (the member S) had not been processed with roughening treatment. In the course of increasing the longitudinal interval size, it was also found that at some level of longitudinal interval size, the adhesion strength was approximately equal to that when the CFRP member (the member S) had not been processed with roughening treatment. In other words, roughening treatment was mean- ingless at some level of longitudinal interval size. [0039] In view of the above, a ratio of the longitudinal interval size to the longest diameter of the recess X was set to be greater than or equal to 25% and less than or equal to 800%, whereby the adhesion strength could be enhanced in comparison with that without roughening treatment for the CFRP member (the member S). [0040] Besides, it is preferable to set the ratio of the longitudinal interval size to the longest diameter of the recess X to be greater than or equal to 50% and less than or equal to 650% in that the adhesion strength could be enhanced by 10% or greater than that without roughening treatment for the CFRP member (the member S). [0041] Furthermore, it is more preferable to set the ratio of the longitudinal interval size to the longest diameter of the recess X to be greater than or equal to 75% and less than or equal to 450% in that the adhesion strength could be enhanced by 20% or greater than that without roughening treatment for the CFRP member (the member S). [0042] Itshould benoted that,regarding a relation between the width-directionalintervalsize and theadhesionstrength, the following was found from the results of another experiment: the width-directional interval size does not exert any effect on the adhesion strength based on the premise that the width-directional interval size is greater than zero. [0043] In addition, the CFRP member S containing unidirectional carbon fibers was used in the experiment described above. However, even in use of a type of CFRP member containing carbon fibers woven in plain or twill pattern in the longitudinal direction, enhancement in adhesion strength can be similarly achieved if the conditions of the experiment described above are satisfied. [0044] This is also true of various types of FRP (Fiber Reinforced Plastics) member using fiber material other than carbon such as glass fiber, ceramic fiber, aramid fiber, aluminum fiber, cellulose nanofiber, or so forth. [0045] Furthermore, it was found as well that, even when the longest diameter and the shortest diameter of the recess X are interchanged, a relation between the adhesion strength and a ratio of the longitudinal interval size to the shortest diameter of the recess X was established similarly to the relation described above. [0046] It should be understood that the embodiment herein disclosed is illustrative only and is not restrictive in all aspects. It is intended that the scope of the present invention is indicated by the appended claims rather than the explanation described above, and encompasses all the changes that come within the meaning and the range of equiv- alents of the appended claims. 1. A laser irradiation device (10) roughening a surface of a member (S) by irradiating a laser beam (L) on the surface of the member (S) so as to enhance adhesiveness on the surface of the member (S), wherein the laser beam (L) is irradiated at intervals of predetermined time, and Claims 6