A batteryless rotary encoder is provided, which includes a rotation detecting section and a signal processing section. The rotation detecting section includes a rotational exciter magnet and a piezoelectric transducer. The piezoelectric transducer is constructed by laminating a magnetic material and a piezoelectric transduction sheet, and the magnetic material faces toward the rotational exciter magnet. When the rotational exciter magnet rotates, the rotational exciter magnet attracts or repels the magnet sheet or the magnetic metal sheet so that the piezoelectric transducer is pressed or stretched by the magnet sheet or the magnetic metal sheet to generate a first output signal. The signal processing section includes a counter and a rectifier. The counter receives the first output signal and calculates revolutions of the first output signal to indicate the number of rotations of the rotational exciter magnet. The rectifier receives the first output signal to power the counter.