A keyboard device for a portable computer is disclosed. The portable computer includes a main body chassis and a display chassis. The main body chassis contains the keyboard device having multiple keycaps elastically supported to be moved up and down. Coupled to the main body chassis via a hinge, the display chassis contains a display. The main body chassis also includes a key depressing mechanism having a rotating shaft member provided along each of the keycaps and a pressing piece projecting from the outer circumferential surface of the rotating shaft member to the side of the keycap to allow a receiving part provided in the keycap to be depressed downward. The rotating shaft member rotates about the axis to cause the pressing piece to push down the receiving part and, in turn, pushing down and keeping the keycap in a depressed position.