There are provided a microchemical system capable of acquiring a highly accurate TLM output value and a method for calculating TLM output thereof. A microchemical system 1 comprises: a microchemical chip having a channel with a depth t in which a sample flows; an exciting light source 13 adapted to irradiate the sample with an exciting light through an objective lens 10 with a numerical aperture NA; a detecting light source 14 adapted to irradiate the sample with a detecting light coaxially with the exciting light through the objective lens 10; and a PD adapted to receive a transmitted light when the detecting light transmits the sample before and after formation of a thermal lens 12. When a TLM output is calculated in the microchemical system 1 on the basis of a received light amount of the PD, the depth t (μm) is set to the range of 75≦t≦300, the numerical aperture NA is set to the range of 0.04≦NA≦0.1 and chromatic aberrations df (nm) for the exciting light and the detecting light of the objective lens 10 is set to the range of 100≦df≦250.