PURPOSE: To easily and quickly perform busy zone changeover (hand off) in a simple portable telephone system.

CONSTITUTION: Even when a portable telephone terminal 7 in the service area of a radio base station 1 (=home area station) roams to the service area of the other radio base station 2, the busy zone changeover is enabled through a separately prepared radio channel (y3). A call goes from the portable telephone terminal 7 to the radio base station 2 (y5), is delivered to a high output transmitter-receiver 5 (y4), is passed through the channel (y3) heading towards the high output transmitter-receiver 4, is received in the radio base station 1 (y2) and is connected to a telephone set 9 via a public network 8 (y1). Even when the portable telephone terminal 7 roams, the channel (y1) is not required to be switched. Compared with the other system, a system price can be substantially reduced and quick hand-off is possible.