An image flow cytometer has a flow chamber with a flow channel formed therein to permit a microparticulate sample to flow through the flow channel. An irradiation optical system irradiates the sample in the channel with incident light in an irradiation spot smaller than a selected representative size, e.g., smaller than the sample. The system scans an irradiation position perpendicular to the flow direction of the sample. A detection optical system is opposed to the irradiation optical system through the flow chamber, or is off the optical axis of the incident light. The detection system detects a light intensity of resultant light from the flow chamber. A control unit detects the microparticulate sample according to a change of the light intensity of the resultant light detected by the detection optical system.