When a reaction liquid in a reaction vessel is irradiated with light to measure a light quantity such as transmitted light, optical axis deviation occurs due to vibration-induced rattling of the reaction vessel, and measurement precision decreases. A torsion coil spring 302 for biasing a reaction vessel 112 is provided in a heat block 200 on which the reaction vessel is mounted and which has a plurality of reaction vessel installation holes 201, the reaction vessel 112 is provided with a projection 501 in a portion of a side surface thereof, and when the reaction vessel is inserted in a hole, an arm part of the torsion coil spring 302 operates so as to retract along the projection, and biases the projection vertically downward and in the horizontal direction when the reaction vessel is settled in the hole, and so as to press the reaction vessel 112 against the wall of the hole 201.