A defective-portion detector has a detecting portion which detects a variation in fuel pressure as a fuel pressure waveform based on a detection value of a fuel pressure sensor and a computing portion which computes, based on the fuel pressure waveform, a plurality of injection-rate parameters required for identifying an injection-rate waveform corresponding to the fuel pressure waveform. Further, the detector has a determining portion which determines whether each learning value of the injection-rate parameters is an abnormal value and an identifying portion which identifies a defective portion in the fuel injection system based on a combination of abnormal learning values which the determining portion has determined.