To provide an inexpensive reaction detector capable of accurately detecting a reaction in a microchip with a camber.

The reaction detector makes a specimen react with a reagent stored in a microchip by injecting liquid for driving into a flow channel formed in the microchip, and optically detects the reaction result of the reagent. The reaction detector comprises a temperature adjusting member for adjusting the temperature of the microchip, a chip pressing member for pressing the microchip and sticks the microchip to the temperature adjusting member, a light emitting means facing a part for measuring the reaction result of the reagent of the microchip, a light receiving element for receiving the reflected light or transmitted light of the light emitting means, and a driving liquid injecting means for injecting driving liquid for driving the reagent in the microchip or the specimen. The chip pressing member is integrated with the light receiving element.