An evaluation device, for evaluating a control unit (ECU) to which another control unit (ECU) is connected through a communication line and communicating with it, which can simulate even an unstable state such as when the other control unit does not start up right after power is turned on. The evaluation device is provided with a communication behavior simulating unit for simulating communication behavior of that other control unit and a switching unit for switching the communication behavior simulating means so as not to simulate the communication behavior. The communication behavior simulating unit is provided with an acknowledgement signal generating unit, and the switching unit disables the operation of the acknowledgement signal generating unit.