A data output device for a vehicle that performs data communication with an external diagnosis tool and performs data communication with an in-vehicle control unit, which outputs specific diagnosis data in response to a request from the diagnosis tool, includes: a cache memory section for storing the specific diagnosis data; a cache save section for controlling the cache memory section to store the specific diagnosis data output from the control unit; and a cache read section for reading the specific diagnosis data in the cache memory section and responding to the diagnosis tool based on the specific diagnosis data when the request from the diagnosis tool requests the specific diagnosis data.