A detection method that can measure a plurality of measured substances in a fluid test sample at the same time, easily perform qualitative analysis and quantitative analysis, and reduce measurement errors between excitation energies of the measured substances, and a microchemical system using the detection method. A probe light with a wavelength of 780 nm CW-oscillated from a probe light source 16 is irradiated on measured substances in a minute channel 1, excitation lights with wavelengths of 658 nm and 532 nm, respectively, are modulated into plurality of flashing excitation lights with frequencies 1 kHz and 1.2 kHz and a duty factor of 50% and irradiated on the measured substances in the minute channel 1, the probe light refracted by a thermal lens formed by the irradiated flashing excitation lights is detected with respect to individual frequency components at the same time, and a signal of the detected probe light is guided from a PD 21 to a PC 24 as a signal processing apparatus via an IV amplifier 22. The PC 24 carries out FFT processing to measure the intensity of the signal with respect to individual frequency components.