PURPOSE: To make the apparatus very compact as a whole by arranging an endless belt body which holds a plurality of reaction containers at each specified interval and a belt driver to transfer the reaction container to an optically measuring position.

CONSTITUTION: An endless belt body 2 is so arranged that protructions 6 on a conveyor piece 3 are fitted into recesses 7 on another conveyor piece to join them together in a belt, eventually forming an endless conveyor flexible. Then, a belt driver B for driving the endless belt body 2 is driven to rotate with a driver. Thus, first and second reagents are distributed to reaction containers 1 moving together with the endless belt body 2 and then, a reaction sample in the reaction container 1 undergoes a colorimetric measurement at an optical measuring position.