Abstract: A key device including a key main body for actuating a mechanical lock device, a knob fixedly secured to the base end of the key main body, and an electronic circuit incorporated in the knob of the key device, wherein: the electronic circuit is carried by a flexible printed circuit board which is folded around an edge of the base end of the key main body and at least partly cover the two major surfaces of the base end. Thus, even when the size of the base end of the key main body is increased so as to improve the resistance of the knob against the force which tends to pry open the knob due to the twisting torque acting upon the key main body during use, through such efficient utilization of the internal space of the knob, the circuity can be fitted into the knob without increasing its size. Furthermore, the base end of the key main body serves as a heat sink and a firm support for the circuit and the accessory component parts.