PURPOSE: To reduce the burden of software of an automatic exchanger, by transmitting the sender information to trunks by the sender transmission section based on the main trunk number re-transmission information at the main trunk transmission number memory section.

CONSTITUTION: In an automatic exchanger, the subscriber requiring the main trunk number automatic re-transmission writes in the main trunk outgoing number to a main trunk transmission number memory section 2 every main trunk transmission, and when the call is imcomplete, the information of the presence of re-transmission request and the information of trunk to be re-transmitted are written in the main trunk transmission number memory section 2 by dialing a special number. Then, a sender control section 3 in a sender transmission section 5 reads out the trunk of the presence of re-transmission request and the main trunk number, and transmits the required information to a main trunk 6 via the sender start section 4. The main trunk 6 judges the information output from the sender start section 4 at a sender information detection section 7 to start the sender relays.