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.