A cassette chamber according to the present invention comprises a housing defining a space stored with a cassette for holding a plurality of objects of treatment, a lift base having a rotatable shaft and located in the housing for up-and-down motion, an auxiliary base fixed to the shaft and inclined at a predetermined angle to the longitudinal direction of the shaf, a cassette support having a bottom support portion set on the lift base and bearing the bottom face of the cassette and a back support portion rotatably supported by the auxiliary base and bearing the back face of the cassette, a rotation mechanism for rotating the shaft as the lift base ascends or descends, thereby rotating the auxiliary base and the cassette support between a first position inside the housing and a second position outside the housing, and a support section for keeping the back support portion of the cassette support parallel to the shaft by engaging the back support portion being rotated to the second position by the rotation mechanism and causing the back support portion to rotate relatively to the auxiliary base.