A jack used in an action mechanism of a piano has substantially an L-shape, composed of a big jack portion and a small jack portion. The thickness of the small jack portion is made smaller than that of the big jack portion in a direction of key arrangement in a state of the jack being fitted in the piano. By reducing the weight of the small jack portion as above, the time required for the jack to return to a position capable of pushing up a part of a striking member can be shortened. Accordingly, more frequent repetitive striking is allowed within a predetermined period.