A transmission system comprising a first communication device and a second communication device connected with each other via a first line and a second line, wherein the first communication device transmits a frame of a post-switching message to the first line after switching a line, used for transmitting frames of VLAN traffic, from the first line to the second line, and the second communication device includes a first buffer which stores the frames of the VLAN traffic received from the first line and a second buffer which stores the frames of the VLAN traffic received from the second line, and when the second communication device receives the frame of the post-switching message from the first line, the second communication device starts reading of the frames of the VLAN traffic from the second buffer after reading all of the frames of the VLAN traffic stored in the first buffer, characterized in that: the first communication device includes a first switching section, a first message processing section connected with the first line, a second message processing section connected with the second line, and a first control section, when the first control section switches the line, used for transmitting the frames of the VLAN traffic, from the first line to the second line, the first control section outputs a pre-switching message transmission command signal to the second message processing section, and after outputting a switching command signal to the first switching section, upon completion of switching by the first switching section, outputs a post-switching message transmission command signal to the first message processing section, the first switching section outputs the frames of the VLAN traffic to the first message processing section in an initial state, and when receiving the switching command signal, outputs the frames of the VLAN traffic to the second message processing section, the second message processing section outputs, to the second line, the frames of the VLAN traffic input from the first switching section after transmitting the frame of the pre-switching message to the second line based on the pre-switching message transmission command, and the first message processing section outputs, to the first line, the frames of the VLAN traffic input from the first switching section, and transmits the frame of the post-switching message to the first line based on the post-switching message transmission command. the first communication device includes a first switching section, a first message processing section connected with the first line, a second message processing section connected with the second line, and a first control section, when the first control section switches the line, used for transmitting the frames of the VLAN traffic, from the first line to the second line, the first control section outputs a pre-switching message transmission command signal to the second message processing section, and after outputting a switching command signal to the first switching section, upon completion of switching by the first switching section, outputs a post-switching message transmission command signal to the first message processing section, the first switching section outputs the frames of the VLAN traffic to the first message processing section in an initial state, and when receiving the switching command signal, outputs the frames of the VLAN traffic to the second message processing section, the second message processing section outputs, to the second line, the frames of the VLAN traffic input from the first switching section after transmitting the frame of the pre-switching message to the second line based on the pre-switching message transmission command, and the first message processing section outputs, to the first line, the frames of the VLAN traffic input from the first switching section, and transmits the frame of the post-switching message to the first line based on the post-switching message transmission command.