To provide an extension device including a patient support face, and an extension beam one end part of which is connected with the patient support face through an interface and which has a holder for a feed shaft structure on the other end part, and including at least two longitudinal beams in which the patient support face supports a pelvis support plate and which are mutually in parallel.

The interface 14 includes: a connecting part 36 on the support face side; and a connecting part 38 on the beam side fixed on the extension beam 16. At least one longitudinal beam 10 of the connecting part 36 on the support face side is supported so that it can be rotated around a shaft vertical to the pelvis support plate, and the connecting part 36 on the support face side is locked to a rotating operation around a relative rotating shaft to the longitudinal beam 10 by a locking device actuated by remote driving.