A heart failure diagnostic device includes a piezoelectric sensor sheet having flexibility and configured to output a detection signal corresponding to an input vibration. A respiratory signal acquisition unit is configured to extract, as a respiratory signal, a signal of a vibration frequency caused by respiration from the detection signal detected by the piezoelectric sensor sheet. A power spectrum calculator is configured to obtain a power spectrum of a respiratory frequency band from the respiratory signal, and a signal corrector is configured to correct the detection signal such that in the power spectrum, a maximum value of a first-order frequency component of a respiratory waveform is not smaller than 1.5 times a maximum value of a second-order frequency component of the respiratory waveform to obtain the respiratory signal.