A diagnostic device for a fuel vapor treatment system having: a canister that captures fuel vapor generated in a fuel tank; a purge passage for purging fuel vapor from the canister to an intake passage of an internal combustion engine; a purge valve provided in the purge passage to adjust a flow rate of fuel vapor; a processing device that depressurize an fuel vapor path extending from the fuel tank to the purge valve via the canister by a pump and performs a diagnostic processing for diagnosing a leak of fuel vapor from the fuel vapor path based on a change in pressure in the fuel vapor path during depressurization; and an angle detection device that detects an angle of inclination of the vehicle. The processing device determines, based on an inclination angle of the vehicle detected by the angle detection device, whether the diagnostic processing can be performed.