To provide a concentration-measuring apparatus which can remarkably simplify setting and discharging operational procedures for a sensor.

A cartridge 2 storing many sheets of sensors 1 in a state urged upward is set in a main body 3 of the concentration-measuring apparatus. The uppermost sensor 1 is pressed by a pressing member 4 to project from the main body 3. A cam member 4b moving following the pressing member 4 and a lever member 4d engaged with the cam member 4b thereby restricting a forward/rearward movement of the cam member 4b are provided in order to set a forward movement length and a rearward movement length of the pressing member 4 to press out approximately half a length of the sensor 1 when the pressing member 4 moves forward at the first time and nearly the entire length of the sensor when the pressing member 4 moves forward at the second time.