A stem is installed to an injector body and is resiliently deformable upon receiving a pressure of high pressure fuel conducted through a high pressure passage of the injector body. A strain gauge is installed to the stem to sense a strain generated in the stem. A molded IC device executes an amplifying operation, which amplifies a signal received from the strain gauge. A retainer is threadably fastened to the injector body and is configured to be rotatable relative to the stem. The stem, the strain gauge the molded IC device are integrally assembled together to form a fuel pressure sensing unit, which is installed to the injector body by threadably fastening a threaded portion of the retainer to the injector body.