In order to obtain a visually high quality image with excellent sharpness using a silver halide photosensitive material, it is necessary to reduce the ratio of EL light in the light output from a GaN based laser diode to 20% or less. For example, when the light intensity measured on a sheet of photosensitive paper is 0.05 mW, the thickness of a waveguide is 3 nm and the width of the waveguide is 3 .mu.m, this condition is satisfied with a length of a resonator being 1 mm or less. In other words, a waveguide width W1 and a resonator length L are set such that the product of the waveguide width W1 and the resonator length L (W1L) becomes 0.003 mm.sup.2 or less. This reduces the output ratio of the EL light, and a high quality image with excellent sharpness can be obtained when a silver halide photosensitive material is exposed.