PURPOSE: To eliminate the pain in a finger and facilitate a long-time musical performance and to obtain a key touch which is closed to that of a piano by putting many granular weight members movably in a storage part which is provided to a mass body and has a closed space.

CONSTITUTION: The mass body 20 formed on a support member (frame) 10 by outsert molding is fitted to a support member rotatably around a fulcrum member 12 for the mass body from its rear part to the front by using the fulcrum member 12. This mass body 20 consists of a resin molding 24 which has plural granular weight storage bodies 21-23 and a mass main body 25 made of an iron material, etc., and part of the mass main body 25 is extended into the resin molding 24 as a reinforcing body and weight assistant body 26 for the resin molding body 24. The storage body 31 for granular weights which is formed by blow molding is provided on the player's-side reverse surface of a key 30. The keys or mass bodies of a keyboard device with the mass bodies may be provided with closed spaces with granular weights.