both protons and neutrons, which indicates that the former configuration favors a prolate shape and the latter one induces a triaxial shape. The SD states in $^{40}$Ar have dominantly the [( sd ) − $^{4}$( pf ) $^{2}$]$_{π}$ [( sd ) − $^{2}$( pf ) $^{4}$]$_{ν}$ configuration, which shows an admixture of shell effects of these configurations. The triaxiality γ values of protons and neutrons of the SD states in $^{40}$Ar are approximately 10 ◦ (Fig. 1), which is the intermediate value between γ ∼ 0 ◦ and 25 ◦ for the SD states in $^{36}$Ar and $^{44}$Ti, respectively. Protons and neutrons may favor similar deformations to increase overlap and gain attractive forces between them. Thus, the triaxiality of the SD states in $^{40}$Ar is understood by a competition of a prolate shape favored by proton shell effects for Z = 18 and a significantly triaxial shape induced by neutron shell effects for N = 22. In conclusion, the structure of the SD states in $^{40}$Ar has been investigated with the deformed-basis AMD and the GCM. The SD states in $^{40}$Ar form triaxial shapes. Owing to the triaxial deformation, the K π = 2 + SD band exists. Calculated B ( E 2) values for intraband transition of the GS and SD0 bands are consistent with experimental data. The triaxiality γ of the SD states is understood by the competition of deformations favored by their proton and neutron structures. To understand details of nuclear structures, it is necessary to take into account triaxiality. Numerical calculations were conducted on the RIKEN Cluster of Clusters (RICC) and the supercomputer SX9 in the Research Center for Nuclear Physics, Osaka University. The authors thank Dr. N. Itagaki in the Department of Physics, Faculty of Science, The Univer- sity of Tokyo and Dr. T. Yoshida in the Center for Nuclear Study, The University of Tokyo, for fruitful discussions. [1] C. E. Svensson, A. O. Macchiavelli, A. Juodagalvis, A. Poves, I. Ragnarsson, S. ˚ Aberg, D. E. Appelbe, R. A. E. Austin, G. C. Ball, M. P. Carpenter, et al., Phys. Rev. C 63 , 061301(R) (2001). [2] E. Ideguchi, D. G. Sarantites, W. Reviol, A. V. Afanasjev, M. Devlin, C. Baktash, R. V. F. Janssens, D. Rudolph, A. Axelsson, M. P. Carpenter, et al., Phys. Rev. Lett. 87 , 222501 (2001). [3] C. D. O’Leary, M. A. Bentley, B. A. Brown, D. E. Appelbe, R. A. Bark, D. M. Cullen, 10