The communication method and the subsidiary transmission apparatus for a point-to-multipoint communication system in which the subsidiary transmission apparatus autonomously establishes point-to-multipoint communication between one main transmission apparatus and multiple subsidiary transmission apparatuses, without manually operating switches of the subsidiary transmission apparatuses. The subsidiary transmission apparatus generates, during the absence of a transmission request to the main transmission apparatus, an alarm signal to be transmitted to the main transmission apparatus through an upstream line, and the main transmission apparatus sends, while receiving the alarm signal through the upstream line, a specified notification signal, which is to be sent out in response to the reception of the alarm signal, to a downstream line, and the subsidiary transmission apparatus evaluates the state of use of the upstream line depending on the presence or absence of reception of the notification signal from the downstream line.