Abstract: A voice monitoring method which is capable of preventing an occurrence of the interruption of a call and reducing the burden of switching software, and also capable of monitoring the contents of a call whose communication path is performed within the same line switch installed in a remote place is provided. A voice from a monitored subscriber telephone set 6-1 is distributed at a line switch 4-1 to a voice monitor path (unidirectional path) 8-1, and a voice from a subscriber telephone set 6-2 of the other party is distributed at the line switch 4-1 to a voice monitor path (unidirectional path) 8-2. The distributed voices are given through a time-division switch 3 to the first and second terminals of a three-way trunk 5, are mixed at the three-way trunk 5, and are sent through the third terminal to a monitor telephone set 7.