In one embodiment of the present invention, a flow cytometer includes a plurality of optical sources, each of which illuminates an excitation beam at a predetermined frequency having a wavelength and a phase different from another; and a guide member for guiding the excitation beams into the same incident optical path to focus onto a dyed particle. According to one aspect of an embodiment of the present invention, it is possible, without requiring introduction of the delay time, to detect a plurality of fluorescence excited at the cell particles labeled with a plurality of fluorescent labeling reagents, by a plurality of laser optical sources.