A protocol conversion device includes a converter and a communicator. The converter makes a protocol conversion of vehicle data acquired from a vehicle by communication based on a first protocol, into vehicle data in a format of HTTP or HTTPS as a second protocol. The vehicle data is directed to diagnosis of the vehicle. The communicator transmits the vehicle data protocol-converted by the converter, to a communication terminal that is able to execute, by communication based on the second protocol, a vehicle diagnosis program held by a web server. The converter makes a protocol conversion of data transmitted from the communication terminal by the communication based on the second protocol, into data in a first protocol format. The communicator transmits, to the vehicle by the communication based on the first protocol, the data in the first protocol format that is protocol-converted by the converter.