PURPOSE: To miniaturize an optical modulator and to prevent luminous flux in the vicinity of an optical axis outgoing from the optical modulator from being chipped.

CONSTITUTION: The optical modulator 10 is provided with a mercury vapor lamp 21, a collimate lens 22, a beam splitter dividing ultraviolet beam UV transmitting through the collimate lens 22 to two beams, a plane mirror 24 reflecting vertically the divided one side ultraviolet beam UV1 and a mirror device 25 reflecting vertically the other ultraviolet beam UV2, and constitutes a so-called Michelson interference device. The mirror device 25 is constituted so that plural optical modulation elements D converting an electric signal to the change (a phase by a half wavelength is changed) of the phase of an optical wave are arranged in a grid shape, and the electric signal in response to the on/off value at every picture element of a graphic pattern required to plot in a PS plate is inputted at every optical modulation element D. The beam outgoing from the mirror device 25 is interred by the beam splitter, and the strength in response to the graphic pattern is added, and the PS plate 12 is exposed.