The behavior of argon plasma generated by a pulse-modulated surface-wave power with an Okamoto cavity was investigated using a time-resolved spectrometer and a Lagmuir probe. Microwave power (2.45 GHz, similar to1 kW) was modulated by a square wave pulse with duration 10-100 mus and 10-80% duty ratio. Time resolved emission spectra of Ar I(811.5 nm, upper state energy E-u = 13.1 eV) and Ar I(419.8 and 355.4 nm, E-u = 14.6 and 15.0 eV, respectively) were obtained as a function of the duty ratio, power and pressure and compared with the probe signals. In the case of Ar I(419.8 and 355.4 nm), the intensity was increased rapidly (spike) just after the microwave power was turned on and just after the power turned off the intensity decayed rapidly. On the other hand, no spike was observed for Ar I(811.5 nm) over the wide pulse range and the intensity decayed more slowly just after the power turned off. The time evolutions of these signals are compared with the time variations of the floating potential (proportional to the electron temperature) and the ion saturation current (electron density) measured by the probe. Good correlation's were obtained between these results. The emission intensity ratio of these lines has a maximum with respect to the duty ratio for constant peak power. The time averaged ion current density and its radial distribution (uniformity) were improved compared with that of continuously driven plasma with the same time-averaged power.