To provide a micro reactor and a micro reactor system that enable measurements related to reaction rate, such as the bonding constant and dissociation constant of a reactant contained in a sample solution, by making the flow speed, i.e. the flow velocity constant.

Fluctuation of the flow velocity or the flow rate of the sample solution is detected by a change of a resonance frequency signal 13 from an electrode 11 formed in a piezoelectric oscillator 10, inside a pressure detecting means 12 arranged in the micro reactor 1, and a pump 17 is drive-controlled, to make the flow velocity or the flow rate of the sample solution constant, by feeding back the fluctuation to the pump 17.