Apparatus for producing timing signals for photometric measurement by utilizing light transmitted through samples to be analyzed. The apparatus comprises a light source, a group of sample containing cells, a driving mechanism for moving the sample cells relative to the light source, a concave diffraction grating for dispersing light transmitted through the sample cell, a first photo-detector for receiving a light beam of a predetermined wave length from the concave diffraction grating, a second photo-detector to receive light beams of the zero spectral order subjected to the total refrection at the concave diffraction grating, a signal hold circuit for holding an electric signal available from the first photo-detector, and a command circuitry which responds to the signal held by the hold circuit to produce signals which resets the hold circuit to release the held signal on one hand and on the other hand allows a read-out and/or record apparatus to read out and/or record the peak value of the signal held by the signal hold circuit. A delay circuit may be provided to delay the operation of the command circuit in order that an analog-to-digital conversion of the peak value signal be effected before the read-out thereof.