To perform communication originated from a portable telephone by the route of the lowest cost.

A telephone line connection system is provided with: the portable telephone 1 used by a caller; a telephone 3 used by a receiver; a caller subscribing side exchange 8 connected to a base station 4a communicable with the portable telephone 1; a most inexpensive line exchange 1 enabling the communication by the route of the lowest communication charge; and a receiver subscribing side exchange 8 connected to the telephone 3 of the receiver. Also, the system is provided with: a telephone number data transmission means 25 for transmitting the data of the telephone number of the most inexpensive line exchange 11 together with the data of the telephone number of the telephone 3 of the receiver; a most inexpensive line exchange connection means 9a for connecting a line to the most inexpensive line exchange 11 on the basis of the data of the telephone number of the most inexpensive line exchange 11; and receiver subscribing side exchange connection means 9a and 11a for connecting the receiver subscribing side exchange 8 while keeping the line with the portable telephone 1 of the caller connected on the basis of the telephone number of the telephone 3 of the receiver.