To provide a novel method for efficiently detecting fluorescent coloring matter having a line detector by spectral analysis.

Spectrum resolving power is measured by the number of individual channels. In such a case that all of the individual channels of a detector are not always read at the same time, a series of reading operations of the individual channels are carried out by multiplexing according to a conventional technique. Multiplexing is performed over the pixel stay time during the scanning of a sample. In this multiplexing method, there is a defect such that the integrating time per each individual channel during a signal detection period is reduced by the number of multiple positions. Further, when a wide band fluorescent spectrum is measured, the signal is lost in the unread individual channel. In another multiplexing method, the signal of the individual channel is relaxed. Next, individual memory devices are alternately read. However, new data is not recorded during reading. Therefore, the reading speed of the line detector is reduced by the multiplexing of this system.