An MCP unit comprising: a microchannel plate arranged on a plane intersecting with a central axis (Ax) of the MCP unit and configured to emit secondary electrons internally multiplied in response to incidence of a charged particle traveling along the central axis, the microchannel plate having an entrance face which the charged particle enters, and an exit face which is opposed to the entrance face and which the secondary electrons exit; a first electrode in contact with the entrance face of the microchannel plate, the first electrode being set at a first potential; a second electrode in contact with the exit face of the microchannel plate, the second electrode being set at a second potential higher than the first potential; an anode arranged as intersecting with the central axis, at a position where the secondary electrons emitted from the exit face of the microchannel plate arrive, the anode being set at a third potential higher than the second potential; an acceleration electrode arranged between the microchannel plate and the anode, the acceleration electrode being set at a fourth potential higher than the second potential and having a plurality of openings for allowing the secondary electrons traveling from the exit face of the microchannel plate to the anode to pass therethrough; and a plurality of insulators having doughnut shape, each arranged between the second electrode and the acceleration electrode; characterized by a ring member arranged between the acceleration electrode and the anode, the ring member being comprised of an insulating material and having a through hole defined by a continuous surface surrounding the central axis so as to allow the secondary electrons traveling from the acceleration electrode to the anode to pass therethrough, so that reflected electrons emitted from the anode in response to incidence of the secondary electrons from the microchannel plate are confined within a space between the acceleration electrode and the anode. a microchannel plate arranged on a plane intersecting with a central axis (Ax) of the MCP unit and configured to emit secondary electrons internally multiplied in response to incidence of a charged particle traveling along the central axis, the microchannel plate having an entrance face which the charged particle enters, and an exit face which is opposed to the entrance face and which the secondary electrons exit; a first electrode in contact with the entrance face of the microchannel plate, the first electrode being set at a first potential; a second electrode in contact with the exit face of the microchannel plate, the second electrode being set at a second potential higher than the first potential; an anode arranged as intersecting with the central axis, at a position where the secondary electrons emitted from the exit face of the microchannel plate arrive, the anode being set at a third potential higher than the second potential; an acceleration electrode arranged between the microchannel plate and the anode, the acceleration electrode being set at a fourth potential higher than the second potential and having a plurality of openings for allowing the secondary electrons traveling from the exit face of the microchannel plate to the anode to pass therethrough; and a plurality of insulators having doughnut shape, each arranged between the second electrode and the acceleration electrode; characterized by a ring member arranged between the acceleration electrode and the anode, the ring member being comprised of an insulating material and having a through hole defined by a continuous surface surrounding the central axis so as to allow the secondary electrons traveling from the acceleration electrode to the anode to pass therethrough, so that reflected electrons emitted from the anode in response to incidence of the secondary electrons from the microchannel plate are confined within a space between the acceleration electrode and the anode.