An image exposing apparatus two-dimensionally scans a printing plate precursor with a light beam modulated based on an image signal to carry out image exposure on the printing plate precursor. The image exposing apparatus includes exposing device for emitting a light beam in a pulsed form with a predetermined pulse repetition period onto the printing plate precursor; branching device for branching at least a part of the light beam emitted from the exposing device; a pulse stretcher for stretching a pulse width of a branch beam branched by the branching device; synchronous signal generator for generating a synchronous signal synchronized with the pulse repetition period of the light beam from the branch beam having the pulse width stretched by the pulse stretcher; and modulating device for modulating the light beam emitted from the exposing device with an image exposure period synchronized with the synchronous signal.