To position an operating table in relation to a diagnostic device easily and quickly.

A device of positioning the operating table includes a guide means is equipped for guiding an operating table 1 movable to a desired direction on a floor surface F of an operating room to a fixed position of the floor surface. The operating table has a base 2 put on the floor surface and a table 4 slidable in one direction on the base, and the base has a caster 5 coming into contact with the floor surface and a telescopic rod 6 to elevate the base on the floor surface in order to contact and release the caster to the floor surface, and the guiding means has a support axis 9 fitted on the floor surface vertically and a pivot arm 10 connecting to the support axis pivotally with the one end and connecting to the base with the other end, and the base is pivoted around the support axis to the fixed position through the pivot arm, and the operating table is positioned to the fixed position when the caster is released from the floor surface by the telescopic rod. The operating table is pivoted at the support axis as a fulcrum so that it can be positioned to the fixed position accurately, easily and quickly with small strength even if the operating table is heavy.