In order to detect a state of a key 30, a differential acceleration sensor 38 is provided in addition to a common position sensor 35 and a velocity sensor 36. A reaction force applied by a solenoid unit 20 is determined on the basis of a function which monotonously increases with respect to a differential acceleration signal j in an initial period which is an early stage of depression of a key. After a lapse of the initial period, the reaction force is determined in accordance with velocity, acceleration and the like, referring to a table. As a result, the reaction force rises up rapidly when a key is depressed strongly.