The present invention intends to reduce the contamination level of an introduction path, miniaturize a circuit board, and improve measurement accuracy by adjusting the number of molecules of a measurement target component to flow into an analysis part, and includes: a detector for detecting the concentration of the measurement target component contained in fluid; an introduction path connected to the detector to introduce the fluid into the detector; and a flow rate switching mechanism adapted to, depending on the concentration of the measurement target component, switch the flow rate of the fluid to be introduced into the detector.