There is disclosed a microchip reactor capable of being used in research on reaction kinetics. With this microchip reactor, the time (reaction time) elapsed until a measurement is made since mixing of two liquids is variable. The microchip reactor comprises a base portion and a movable portion which have sliding contact surfaces that are in contact with each other. The base portion and movable portion have their respective downstream flow channel portions which meet at an intersection. Different reagents are introduced at the intersection, mixed, and reacted. An opening is formed in the movable portion. A slot is formed in the base portion. The major axis of the slot is in the direction of sliding motion of the movable portion. The position of the intersection where the reagents are mixed is determined by the intersection of the opening and the slot.