A fuel injection system for use with an internal combustion engine supplies fuel to an injector (fuel injection valve) from a common rail (accumulator) through a high-pressure pipe to spray the fuel from a spray hole formed in the injector. A thin-walled portion is formed in a path member (e.g., an injector body, the high-pressure pipe, or a connector connecting the injector and the high-pressure pipe) and defined by a locally thin wall of the path member. A strain gauge (strain sensor) is affixed to the thin-walled portion to measure strain of the thin-walled portion arising from the pressure of fuel in a high-pressure fuel path.