To provide a sound-proof and vibration-proof joist for floor installment corresponding to an extremely low floor without impairing strength of a vibration-proof joist for a dry-type sound-proof double floor which is generally used.

A part of a ligneous part 4 at which a support leg 1 is continuously provided is cut off to attach the support leg 1 to the ligneous part 4 for corresponding to the extremely low floor 1. A metal fitting 3 formed of a metal plate having a nut through hole 6 is embedded to the part 5 in the vibration-proof joist B formed of a ligneous parallel ply wood such as a LVL which is generally used. A bottom surface of the embedded metal plate receives a bottom surface of the ligneous part of the vibration-proof joist, so that weakness of the strength caused by the cutting-off of the ligneous part 4 is supplemented.