A keyboard apparatus includes plural keys, electronic actuators, reflection plates, and optical sensors. Each key extends in the longitudinal direction and pivots in the vertical direction about a support in accordance with key depression and release. Each electronic actuator has a movable member that displaces vertically when interlocking with a key's pivot movement so as to apply a reaction force against key depression. Each plural reflection plate is fixed to the electronic actuator's movable member, wherein the reflection surface faces the lateral direction of each of the keys. The light reflectance changes along the displacing direction of the movable member. Each optical sensor is arranged apart from the longitudinal axis of the keys in the lateral direction. It emits light toward the reflection plate and receives the reflected light from the reflection plate so as to output an electric signal according to the quantity of received light.