A sensor insertion assembly includes: a sensor insertion device comprising a needle member configured to be inserted in a living body together with a biological information sensor and to be pulled out of the living body after leaving a distal end side of the sensor in the living body; and a base plate detachably attached to a first end side of the sensor insertion device, the base plate comprising a clamp portion configured to move so as to clamp a proximal end side of the sensor extending to an outside of the living body after pulling out the needle member and along with an operation of detaching the sensor insertion device from the base plate.