Abstract: In a tissue metabolism measuring apparatus, light sources (31 to 33) emit rays of different wavelengths; the rays are branched as reference light pulses and sample light pulses through optical fibers (38 and 39); the sample light pulses are transmitted through a living body to be examined; the reference light pulses are delayed by means of mirrors (48 to 50); the sample light pulses transmitted through the living body and the delayed reference light pulses are collected by a lens (44); based on the collected light, a second harmonic of the light is generated by an optical crystal (45); and the second harmonic enters a photomultiplier (72). A photon counter 52 counts photons based on the output of the photomultiplier and calculates an average value of the count output.