Providing an antenna device and a portable radio communication device whose conductive plate for use in reducing the amount of the electromagnetic waves to be absorbed into a human body can be reduced in size. The portable radio communication device 1 includes a circuit board (not shown) necessary for performing radio communication, shield case 2 as a ground conductor which shields the circuit board, a conductive plate 3, an antenna feeding portion 4, and an antenna 5. The circuit board, shield case 2, and conductive plate 3 are enclosed by a housing (not shown) made of nonconductive material. The conductive plate 3 has its one end along the longitudinal direction connected to the shield case 2 to form a short circuit via the conductor 7, and has its other end electrically opened from the shield case 2. The conductive plate 3 has two slits 8a, 8b near the conductor 7.