PURPOSE: To improve the operability by using only a master telephone number section to attain mutual connection, holding and switching connection among a caller line, a secretary telephone line and a master telephone line respectively.

CONSTITUTION: An incoming call given to the master telephone set 121 is received by a master button 222 of a secretary telephone set 122 for response and the incoming line is held by the 2nd operation of the same master button 222 to call the master line. When the master telephone set 121 responds to the call and the secretary telephone set 122 is brought into hook-on state at talking state, the holding of the incoming line is released, the line is connected to the master telephone set 121 for talking state and the secretary service of transfer is completed.