A key unit (100) that locks and unlocks a target by transmitting a signal to a locking and unlocking device (300), includes: a substrate on which a power supply circuit and a reception circuit that receives a request signal transmitted in a first frequency band from the locking and unlocking device (300) via a reception antenna are mounted; and a housing configured to accommodate the substrate. Among parts associated with the power supply circuit, a part having the highest height from a substrate surface is mounted on a first surface of the substrate. The reception antenna is mounted on a second surface. When the substrate is accommodated in the housing, a distance from a substrate surface of the second surface to an inner surface of the housing is shorter than a distance from the substrate surface of the first surface to the inner surface of the housing.