An action for a piano is provided for clarifying a timing of let-off, improving the sequential touching capabilities, and consequently realizing high playing performance excellent in expressive power in a simple structure without adversely affecting the key touch feeling. The action for a piano swings a hammer in response to a touch on a key, and comprises a wippen pushed up by the key to pivotally move, a regulating button having a flat jack contact surface on the lower end, and a jack having a flat regulating button contact surface which comes into contact with the jack contact surface as the key is depressed. The jack comprises a molding made of a thermoplastic resin containing long fibers for reinforcement.