A sensor apparatus includes multiple pairs that each include a light emitting element and a light receiving element configured to receive light emitted from the light emitting element, and, in response to optical axes formed in a region between the light emitting elements and the light receiving elements entering a blocked state, senses a sensing target moving in the region. In an initial state, sensing is performed using a sensing optical axis composed of a portion of the optical axes, and when an optical axis of the sensing optical axis used in sensing in the initial state is blocked, the number of optical axes included in the sensing optical axis is increased so as to expand the sensing optical axis.