A flow cytometer including a flow cell in which an imaging object flows; a laser beam irradiator configured to radiate laser beam; a camera including an image sensor of N×M pixels; and an optical system configured to introduce the laser beam from the laser beam irradiator to imaging range of flow cell and to introduce signal light, such as transmitted, reflected or scattered light, from imaging range of flow cell, to camera. Optical system includes a mirror device that is placed on a Fourier plane of imaging optical system, that has at least one mirror specularly reflecting the signal light, and that is driven and rotated in conjunction with a flow in flow cell, such that each part of an image formed by the signal light is introduced into an identical pixel of the image sensor for at least a predetermined time period from a predetermined timing.