Vehicle operation diagnostic devices, methods, and programs detect an idling stop of a vehicle. The devices, methods, and programs acquire a fuel consumption required for an engine of the vehicle to start, and acquire, if the idling stop is detected, an amount of fuel consumed if idling were to be continued during a period in which the idling stop is performed. The devices, methods, and programs determine whether a fuel consumption reduction effect is obtained based on the engine-start fuel consumption and the fuel consumption if idling were to be continued, and provide guidance based on the determination of whether the fuel consumption reduction effect is obtained.