To provide a key device for improving commodity property and capable of confirming a locking state of a door by providing a memory for storing an identification code, input/output parts and display elements, and displaying residual quantity of fuel and locking/unlocking information.

A key device is composed of a memory 9, input/output parts 11, 13 and display elements 6, 15G, 15R. The memory 9 stores a first identification code signal for permitting starting of an engine and a second identification code signal for locking/unlocking a door, a liquid crystal display element 6 is constituted so as to display residual quantity of fuel, a terminal part 11 inputs fuel residual quantity data, and outputs the first identification code signal. A transmitting/receiving part 13 is constituted so as to transmit the second identification code signal and to receive a lock state signal. When installing the portable key device 5 in a vehicle side input/output device, the first identification code signal is outputted, and the fuel residual quantity data is inputted at any time. A green light emitting diode 15G is lighted in a locking state, and a red light emitting diode 15R is lighted in an unlocking state. Thus, commodity property can be improved.