Vehicle operation diagnostic device, method, computer program and computer readable medium

A vehicle operation diagnostic device comprising an idling stop detection unit (41) that detects an idling stop of a vehicle, an engine-start fuel acquisition unit (41) that acquires an engine-start fuel consumption, a tentative fuel acquisition unit (4) that acquires, if the idling stop detection unit (41) detects the vehicle performing the idling stop, a tentative fuel consumption that is an amount of fuel consumed when idling is continued during a period in which the idling stop is performed, a reduction effect determination unit (41) that determines whether a fuel consumption reduction effect is obtained by the vehicle performing the idling stop compared to continuing idling, based on the engine-start fuel consumption and the tentative fuel consumption, and a determination result guidance unit (41) that provides guidance based on a determination result of the reduction effect determination unit (41).