Provision of an intermediate shaft and a torque meter between a test product and a dynamometer in a transmission testing device in which the test product is set in an environmental chamber leads to a long axial length and thereby requires a large floor area for setting, and also causes a twisting of a shaft and leads to a poor rigidity. A hollow spacer is provided between a face plate of the environmental chamber and a test product attachment holder to which the test product is attached. A flange with an H-shape in side view is disposed in the hollow of the spacer. The flange has an end connected to an adaptor flange fixed to the test product, and another end connected to a rotating shaft of the dynamometer via the torque meter.