This detector includes: a filtering unit performing a first filtering process of performing a filtering process on a first waveform or a second filtering process of performing a filtering process on a second waveform; a detection unit detecting a timing of a first peak value corresponding to a predetermined peak value of a third waveform; a correction unit which corrects a deviation of a detection value corresponding to a timing detected by the detection unit with respect to a timing when the fuel injection valve is actually opened or closed based on a difference between the first peak value of the third waveform and a second peak value of the third waveform appearing after the first peak value; and a detecting unit which detects one or both of the opening and closing of the fuel injection valve based on the detection value corrected by the correction unit.