To provide a vapor pressure measuring instrument of fuel, preventing the deterioration of a detection means, and reducing power consumption.

An engine control system 10 is equipped with: a fuel tank 20; a fuel pump 26 for supplying the fuel in the fuel tank 20 to an injector 12; a fuel vapor producing part 40 for vaporizing the fuel; a first fuel passage 22 for connecting the fuel pump 26 and the injector 12; the second fuel passage 23 connected to the fuel pump 26 at its one end and connected to the fuel vapor producing part 40 at its other end; a pressure sensor 46 for detecting the pressure in the fuel vapor producing part 40; and a fuel temperature sensor 48 for detecting the temperature of the fuel supplied to the fuel vapor producing part 40. An ECU 30 calculates lead vapor pressure RVP on the basis of the respective detection values of the pressure sensor 46 and the fuel temperature sensor 48 and stores the RVP, and calculates the vapor pressure VP of the fuel on the basis of the stored lead vapor pressure RVP and the temperature of cooling water or the temperature of the fuel at that time.