A transmission device for alternately performing transmission and reception to/from a communication destination transmission device, the transmission device comprising: transmission means that transmits transmit data to the communication destination transmission device; reception means that receives return data transmitted by the communication destination transmission device after the transmit data reaches the communication destination transmission device; monitor means that monitors a voltage value of the received return data; an equalizer that amplifies the voltage value of the received return data at an adjustable amplification rate; and control means that, treats a predetermined period of time as one time unit, tries one transmission of transmit data and one reception of return data within the one time unit, and in the case of determining that the one transmission of transmit data and the one reception of return data do not fall within the one time unit, controls the transmission device such that N number of transmit data are continuously transmitted and then N number of return data are continuously received within N, N is an integer of two or more, time units, wherein the transmission device is adapted to adjust a setting value of an amplification rate of the equalizer while confirming whether an output voltage value of the equalizer is within a predetermined range, and to determine whether the one transmission of transmit data and the one reception of return data falls within the one time unit, and when the one transmission of transmit data and the one reception of return data do not fall within the one time unit, what the number of N is, depending on a setting value of the amplification rate of the equalizer when the output voltage value of the equalizer is within the predetermined range. transmission means that transmits transmit data to the communication destination transmission device; reception means that receives return data transmitted by the communication destination transmission device after the transmit data reaches the communication destination transmission device; monitor means that monitors a voltage value of the received return data; an equalizer that amplifies the voltage value of the received return data at an adjustable amplification rate; and control means that, treats a predetermined period of time as one time unit, tries one transmission of transmit data and one reception of return data within the one time unit, and in the case of determining that the one transmission of transmit data and the one reception of return data do not fall within the one time unit, controls the transmission device such that N number of transmit data are continuously transmitted and then N number of return data are continuously received within N, N is an integer of two or more, time units, wherein the transmission device is adapted to adjust a setting value of an amplification rate of the equalizer while confirming whether an output voltage value of the equalizer is within a predetermined range, and to determine whether the one transmission of transmit data and the one reception of return data falls within the one time unit, and when the one transmission of transmit data and the one reception of return data do not fall within the one time unit, what the number of N is, depending on a setting value of the amplification rate of the equalizer when the output voltage value of the equalizer is within the predetermined range.