A fuel delivery device 11 has a delivery pipe 18, which supplies fuel to a fuel injection valve 21 of an internal combustion engine 10. An electromagnetic relief valve 22 releases the fuel from the delivery pipe 18 in response to an opening instruction and lowers the pressure of the fuel in the delivery pipe 18. A diagnosis device for the relief valve 22 has an electronic control unit 27 outputting the opening instruction to the relief valve 22 in response to a stopping instruction for stopping the engine 10. The unit 27 determines whether the relief valve 22 has a defect based on a manner in which the pressure of the fuel in the delivery pipe 18 changes after output of the stopping instruction. As a result, it is appropriately diagnosed whether the electromagnetic relief valve 22 has a defect.