An insulating tube has one end and another end. An An avalanche photodiode (APD) is provided outside the one end of the insulating tube. The another end of the insulating tube is air-tightly connected to an outer flange through a stem inner wall. Capacitors electrically connected to the APD are provided in the insulating tube. The capacitors remove direct current components from signals that the APD generates when detecting electrons. By providing the capacitors in the insulating tube, response of output signals can be prevented from being impaired.