A fuel-pressure waveform detector has a detect-waveform obtaining unit for obtaining a multi-stage injection pressure waveform by means of a fuel-pressure sensor while performing a multi-stage fuel injection during one combustion cycle. A model waveform memory stores a reference model pressure waveform of when a single fuel injection is performed. A waveform extracting unit extracts a pressure waveform due to the subject fuel injection by subtracting the reference model pressure waveform from the multi-stage injection pressure waveform. A correction unit corrects the reference model pressure waveform in such a manner that its attenuation degree becomes larger as a fuel injection period of the subject fuel injection is longer.