A communication control method performing Discovery processing, which is a procedure at an OLT to detect an ONU newly connected, in a PON system, the method includes: a transmission-permission-signal transmitting step of transmitting, by the OLT, a transmission permission signal for discovery, which includes an individual number of an ONU that is permitted to respond and mask information for designating a match-detection target bit for the individual number; and a registration-request-signal transmitting step of comparing, by an ONU, which is not registered in the OLT, a match-detection target bit for the individual number designated in the mask information with an individual number of the ONU based on a received transmission permission signal, and when the target bit and the individual number match each other, transmitting a registration request signal to the OLT.