It is equipped with an injector body 4z which has formed therein high-pressure paths 6az, 6bz, and 6cz through which high-pressure fuel flows to a spray hole and stores therein a piezo-actuator 2z (i.e., an opening/closing mechanism) and a back-pressure control mechanism 3z (i.e., an opening/closing mechanism) which open or close the spray hole, a stem 51z (i.e., an elastic body) which is installed in said body 4z and elastically deformed when subjected to pressure of the high-pressure fluid, and a strain gauge 52z (i.e., a sensing device) which converts the degree of strain occurring at the stem 51z into an electric signal and outputs it as a measured-pressure signal. Specifically, the stain gauge 52z is installed on the stem 51z that is constructed as being separate from the body 4z.