A motive power transmission adapter includes a casing and at least one power transmission portion. The casing is disposed between a surgical tool and a power unit for driving the surgical tool. In addition, the casing includes a clean surface which is a surface facing the surgical tool disposed in a clean region and an unclean surface which is a surface facing the power unit disposed in an unclean region. At least one power transmission portion is movable relative to the casing and transmits a movement of the drive portion to the driven portion. In addition, at least one power transmission portion is disposed to be movable in a direction in which the clean surface and the unclean surface extend and is disposed between the drive portion and the driven portion in a direction intersecting a linear motion direction of the drive portion.
