The level spacings of the J π ≤ 6 + states in the calculated GS band are rotational, but the level spacings between the J π = 6 + and 8 + are smaller than those in the case of a rotational band (Fig. 4). The 2¯ hω [(Δ N π H . O $_{.}$, Δ N ν H . O $_{.}$) = (1 , 1)] components mix in the J π = 8 + state; that breaks the rotational spectra. The calculated GS band deviates from the experimental one and is more rotational. The experimental GS band may be reproduced by adding sufficient basis wave functions such as (Δ N π H . O $_{.}$, Δ N ν H . O $_{.}$) = (1 , 1) components. The observed J π = 2 + 3 (3.21 MeV) and 2 + 4 (3.51 MeV) states were not obtained perhaps because basis wave functions with a total of 2¯ hω configurations are insufficient in the present basis set. Table I shows theoretical and experimental values of electric quadrupole transition strengths B ( E 2) in the Weisskopf unit. The B ( E 2) values for the intraband transitions of the SD0 and SD2 bands take larger values than those of the GS band, which reflects the large deformations of the SD0 and SD2 bands. The B ( E 2) values for the intraband transitions of the GS and SD0 bands are consistent with experimental data. Since the SD0 and SD2 bands are approximately regarded as the K π = 0 + and 2 + bands associated with the triaxial superdeformation, the interband transitions between them have non-negligible values. The observed J π = 2 + 5 (3.92 MeV), 3 + (4.23 MeV), and (2 , 3 , 4) + (5.17 MeV) de- cay to the SD0 band members. The agreement of the theoretical and experimental B ( E 2) values of the 2 + SD2 → 0 + SD0 transition supports the band assignment of the SD2 band for the J π = 2 + 5 (3.92 MeV) state. B ( E 2) ratios of B ( E 2; 2 + SD2 → 0 + $_{SD0}$) /B ( E 2; 2 + SD0 → 0 + $_{SD0}$) and B ( E 2; 2 + SD2 → 2 + $_{SD0}$) /B ( E 2; 2 + SD0 → 0 + $_{SD0}$) are consistent in the case of γ ∼ 10 ◦ with the Davydov-Filippov model [25], which is a schematic triaxial rotor model. To confirm the proposed band assignment of the SD2 band and triaxial deformation of the SD states, more experimental data for intraband transitions of the SD2 band and interband transitions between the SD0 and the SD2 are required. To discuss shell effects of protons and neutrons in the SD states of $^{40}$Ar, triaxial aspects of the superdeformation in $^{40}$Ar[( Z, N ) = (18 , 22)] are compared with those in the N = Z isotope ( $^{36}$Ar[( Z, N ) = (18 , 18)]) and isotone ( $^{44}$Ti[( Z, N ) = (22 , 22)]). The deformed- basis AMD and HF calculations suggest that the SD states in $^{36}$Ar [6, 13] and $^{44}$Ti [6, 9] form prolate and significantly triaxial shapes, respectively. The theoretically suggested triaxiality γ values of the SD states in $^{44}$Ti are approximately 25 ◦ [9] by the deformed- basis AMD, which is consistent with the HF [6]. The particle-hole configurations of the SD states in $^{36}$Ar and $^{44}$Ti are considered to be [( sd ) − $^{4}$( pf ) $^{2}$] and [( sd ) − $^{2}$( pf ) $^{4}$], respectively, for 8