Abstract: The present invention intends to generally maintain the microplate having added test samples in wells therein at even temperature for measuring biochemical or biological reaction accompanying changes in absorbance such as enzyme reaction. Monochromatic lights with various wave lengths from a light source 3 are transmitted through the test samples added to a plurality of wells in a microplate 1 to measure absorbance of the test sample. For this end, a metal plate 2 with good heat conductivity is substantially contacted with the bottom of each well in the microplate 1 and a perforated board 7 is disposed with an air space over the microplate 1 at the starting position thereof. Warmed air heated by a heater 17 is blown onto the microplate 1 through the perforated board 7 and is circulated along the metal plate 2 within a chamber 9.