To provide a diagnostic program for a brake liquid leak after collision.

This diagnostic device comprises detecting means mounted on a vehicle for detecting a vehicle collision damping and an acceleration; first control means for outputting a control signal, depending on largeness of an impact strength exerted to a vehicle, based on information detected by the detecting means; and a brake control means for starting a previously stored control program, based on a control signal outputted by the first control means, and executing and processing running control of each electric control unit provided on the vehicle, according to the control program. The brake control means is provided with a diagnostic function for inconvenience occurrence, and previously stores at least one or more brake leak diagnostic program started by the control signal from the first control means, depending on the largeness of the impact strength of collision.