To enable such the state that the velocity of a test vehicle does not reach a target velocity due to a mechanical wear when being decoupled, to be detected beforehand.

A controller 23 applies current to a DC motor 13 which drives a rope winch 12, whereby a traction force is applied to the test vehicle C being on a stop position S. Then, a period of time for which the vehicle C glides on a runway r and passes through a release position R, is processed such that an actual running period of time (actual arrival value) is compared with a theoretical running period of time (theoretical arrival value) computed in the controller 23, thereby detecting the lowering of the target velocity at the decoupling.