EP 3 800 000 A1 laser light source 12 and the light collector 14 in combination. [0032] The focal positions F of the laser beams L irradiated on the member S from the respective laser light sources 12 are set to be aligned in a row at predetermined intervals. [0033] The laser irradiation device 10 is caused to irradiate the laser beams L atintervals of predetermined time, while being entirely moved in a direction orthogonal to the aligning direction of the focal positions F (hereinafter, the aligning direction willbereferred to as"widthdirection",whereasthe direction orthogonalthereto willbe referred toas "longitudin al direction").Accordingly,the recessesX canbe formed in a gridpattern on the surface T ofthe memberS asshown in FIG.7. [0034] With the laser irradiation device 10, the laser beams L were irradiated on the surface T of CFRP member (the member S) made of a large number of carbon fibers extending approximately in parallel alignment with each other. In this condition, the following parameters are set: the diameter of each recess X formed by each laser beam L; width- directional interval size, i.e., the size of interval between width-directionally adjacent focal positions F in which adjacent laser beams L are irradiated; and longitudinal interval size, i.e., the size of interval between longitudinally adjacent focal positions F in which each laser beam L is irradiated. In this setting, a preferred value for the longitudinal interval size was examined. It should be noted that the extending direction of the carbon fibers contained in the CFRP member (the member S) is herein matched with "longitudinal direction". [0035] The member S is fixed, whereas the laser irradiation device 10 is caused to irradiate the laser beams L with a pulse-like waveform on the member S at intervals ofpredetermined time,whilebeing movedin the longitudinaldirection. Because of this, each recess X is made in the shape of an ellipse elongated in the longitudinal direction as seen in a plan view. In the following explanation, when the recess X is seen in the plan view, the longitudinal length thereof will be referred to as "longestdiameter",whereas the width-directionallength thereofwillbereferred toas"shortestdiameter". [0036] In the present experiment, "longest diameter" is set to 0.1 mm, whereas "shortest diameter" is set to 0.05 mm. Besides, post-roughening adhesion strength on the CFRP member (the member S) was examined, where the width- directional interval size is made constant at 0.05 mm, whereas the longitudinal interval size is made variable. [0037] The CFRP member (the member S) and a round rod are adhered by epoxy adhesive XNR3324/XNH3324 (two-part liquid) manufactured by Nagase ChemteX Corporation). The round rod is herein made of stainless steel with a diameter of 10 mm and the surface thereof had been processed with sandblasting (with grit #180 for 5 seconds at distance of 10 mm). The CFRP member (the member S) and the round rod were left in this condition for 0.5 hours at 100 Celsius degrees. The round rod was then pulled to be separated from the CFRP member (the memberS). Atthe point oftime of separation,a force (N)was measured as adhesion strength. Experimental results are shown in TABLE 1 and FIG. 8. [TABLE 1] LONGITUDINAL INTERVAL SIZE  (mm) RATIO OF LONGITUDINAL INTERVAL SIZE TO  LONGEST DIAMETER ADHESION  STRENGTH (N) 0 0% 500.74 0.025 25% 704.24 0.05 50% 864.8 0.075 75% 1006.86 0.1 100% 1119.38 0.15 150% 1124.72 0.2 200% 1116.645 0.25 250% 1119.62 0.3 300% 1082.44 0.35 350% 1076.88 0.4 400% 1 002.46 0.45 450% 945.66 0.5 500% 927.48 0.55 550% 900.65 5 (Experimental Example)