A communication monitoring system comprising: a master equipment and a slave equipment, wherein the master equipment is provided with a master base station, a master station, a network hub, and a history management server, wherein the master base station is configured to communicate with each of the master station and the history management server in a wired manner via the network hub, wherein the slave equipment is provided with a slave station, a further network hub, and a slave base station, wherein the master base station and the slave base station communicate with each other in a wireless manner, wherein the slave base station is connected to the slave station in the wired manner via the further network hub, wherein the master base station and the slave base station function as transmission devices that transmit a communication frame communicated between the master station and the slave station in the wired manner and in the wireless manner, and wherein the history management server is configured to record, as preceding/succeeding history information, communication history information of a communication frame communicated by each of the master base station and the slave base station before and after a missing communication frame, wherein the master station and the slave station each communicate an individually set, fixed number of communication frames in a cyclic period, wherein the communication frame contains, as communication information, a sequence number and a period number, the sequence number indicating an order in the cyclic period, the period number incrementing each time the cyclic period passes, and wherein the master base station and the slave base station determine, upon communication of each communication frame in the wireless manner, if there is a missing communication frame on a basis of the communication information of a communication frame that has been received this time in the wireless manner and is called, in the following, this-time communication frame, and the communication information of a communication frame that was received last time in the wireless manner and is called, in the following, last-time communication frame, and when there is a missing communication frame, transmit communication history information of the last-time communication frame and of the this-time communication frame to the history management server, as the preceding/succeeding history information to be used for failure cause analysis. a master equipment and a slave equipment, wherein the master equipment is provided with a master base station, a master station, a network hub, and a history management server, wherein the master base station is configured to communicate with each of the master station and the history management server in a wired manner via the network hub, wherein the slave equipment is provided with a slave station, a further network hub, and a slave base station, wherein the master base station and the slave base station communicate with each other in a wireless manner, wherein the slave base station is connected to the slave station in the wired manner via the further network hub, wherein the master base station and the slave base station function as transmission devices that transmit a communication frame communicated between the master station and the slave station in the wired manner and in the wireless manner, and wherein the history management server is configured to record, as preceding/succeeding history information, communication history information of a communication frame communicated by each of the master base station and the slave base station before and after a missing communication frame, wherein the master station and the slave station each communicate an individually set, fixed number of communication frames in a cyclic period, wherein the communication frame contains, as communication information, a sequence number and a period number, the sequence number indicating an order in the cyclic period, the period number incrementing each time the cyclic period passes, and wherein the master base station and the slave base station determine, upon communication of each communication frame in the wireless manner, if there is a missing communication frame on a basis of the communication information of a communication frame that has been received this time in the wireless manner and is called, in the following, this-time communication frame, and the communication information of a communication frame that was received last time in the wireless manner and is called, in the following, last-time communication frame, and when there is a missing communication frame, transmit communication history information of the last-time communication frame and of the this-time communication frame to the history management server, as the preceding/succeeding history information to be used for failure cause analysis.