In an optical switch device, an optical control pulse is inputted into a nonlinear optical fiber in phase with each optical pulse of signal light which is to be outputted, so that the polarization directions of the optical control pulse and the signal light form a predetermined angle at the input stage of the nonlinear optical fiber, the optical control pulse causes polarization rotation and parametric amplification of the optical pulse in the nonlinear optical fiber, and then the optical pulse is outputted through a polarizer. The excitation level of the optical control pulse is set on the basis of the difference of a monitored output level of the optical pulse from a target level so that the output levels of optical pulses of the signal light are controlled at one or more desired levels.