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The valence shell excitations of argon were investigated by an angle-resolved fast-electron energy-loss spectrometer at an incident electron energy of $2500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the transition multipolarities for the excitations of $3p\ensuremath{\rightarrow}3d$, $4d$, $5s$, and $5p$ were elucidated with the help of the calculated intermediate coupling coefficients using the COWAN code. The generalized oscillator strengths for the excitations to $3{p}^{5}(3d,3{d}^{\ensuremath{'}})$, $3{p}^{5}(5p,5{p}^{\ensuremath{'}})$, and $3{p}^{5}(5s,4d)$ were measured, and the profiles of these generalized oscillator strength were analyzed. Furthermore, although the present experimental positions of the maxima for the electric-monopole and electric-quadrupole excitations in $3p\ensuremath{\rightarrow}5p$ are in agreement with the theoretical calculations [Amusia et al., Phys. Rev. A 67, 022703 (2003)], the generalized oscillator strength profiles show obvious differences. In addition, the experimental generalized oscillator strength ratios for the electric-octupole transitions in $3p\ensuremath{\rightarrow}3d$ are different from the theoretical prediction calculated by the COWAN code.