A physical quantity detecting device (10) detects a plurality of different physical quantities on the basis of an output signal waveform. A strain sensor (3) as one sensor element outputs a sensor signal waveform (15) having a reference level (151), a positive level positively changing from the reference level (151), and a negative level negatively changing from the reference level (151). An estimating unit (4) estimates a first physical quantity corresponding to a peak value (152) of the positive level and a second physical quantity corresponding to a peak value (153) of the negative level on the basis of the sensor signal waveform (15) output by the strain sensor (3).