Disclosed herein is a microchip provided with a specimen flow pass, a reagent flow pass, a confluence flow pass all of which are fine flow passes. The specimen flow pass allows specimen to flow toward one end thereof. The reagent flow pass is connected to that end of the specimen flow pass and allows at least one reagent to flow for reaction with the specimen. The confluence flow pass extends from that end of the specimen flow pass and allows the confluent specimen and reagent to flow. A sensing portion is assigned near or at the confluence flow pass, and the reaction of the specimen and the reagent is capable of being detected or observed there. Further to this, the microchip comprises a force applying means for reciprocally moving the specimen and the reagent at the sensing portion. Since the reaction of the specimen and the reagent is occurred while they are reciprocally moved within the sensing portion, a length of confluence flow pass required for the reaction can be shortened, and therefore, a size of the microchip can be made compact.