Keyless entry system

In a keyless entry system (100) including an in-vehicle device (10), and a portable device (20) operated by a built-in battery (19), a request signal is transmitted from the in-vehicle device (10) to the portable device (20) so that wireless communication is performed between the in-vehicle device (10) and the portable device (20), the portable device (20) can set first sensitivity that is predetermined sensitivity and second sensitivity that is higher sensitivity than the first sensitivity and at which a detection area for the request signal is wide, as detection sensitivity for detecting the request signal, sets the detection sensitivity to the second sensitivity when determining that the portable device (20) is in the vehicle (50)'s interior (51) or within the predetermined distance from the vehicle (50), and sets the detection sensitivity to the first sensitivity when determining that the portable device (20) is not in or comes to be not in the vehicle's interior (51) or within the predetermined distance from the vehicle (50).