(57) There is provided a logic device (2, 12, 13) wherein an alternating input signal having a frequency which is one of two frequencies allotted to two input logical values, respectively, positive logic "1" and negative logic "0", is inputted to the device. An output logical value is decided depending on the determination which frequencies the input frequency coincides with or not, and an alternating output signal having a frequency which corresponds to the output logical value is delivered out of the device. The logic device, even when constituted with semiconductor elements, can have highly reliable fail-safe capability and can be reduced in size and weight.