To provide an engine control system and a communication connection device enabling automatic start and/or stop of an engine during vehicle stop.

GW10 is provided with a control part 11 composed of CPU. The control part 11 functions as a start request information generation means and a stop request information generation means requesting engine control. Start (stop) request information generated by the start request information generation means (stop request information generation means) is sent to an engine system ECU 30 controlling start and stop of the engine via LAN 4a. The engine system ECU 30 receives start (stop) request information and starts (stops) the engine.