Spectroscopic measurements of electron temperatures(T_e) were carried out for the shock wave plasmas, generated in a electromagnetic shock tube, with low pressure argon as the test gas. The T_e's were estimated by the Boltzmann plot method with six spectral lines of singly-ionized argon, for every 0.2μsec after the arrival of plasma to the observation port. Typically, the T_e rises to the peak value of 〜4×10^4K in 〜10μsec, after the arrival of plasma, then rapidly decreases to the steady value of 〜1.5×10^4K during observation period. The rapid decrease of T_e seems to correspond to the rapid increase of electron number density.