To use a frequency capable of reducing attenuation of an electric signal to be transmitted, by controlling the transmission speed of the electric signal in a transmission line.

This apparatus is equipped with a master unit 10 with a monitor and a slave unit 20 with a camera connected with a transmission line L, and is provided with a power-source activation detector 101 for the master unit with a monitor which detects that the power source of the master unit with a monitor is turned on, a power-source activation detector 201 for the slave unit with a camera which detects that the power source of the slave unit with a camera is turned on, a pseudo-random pattern generating circuit 102 for generating a pseudo-random pattern signal when the power-source activation detector for the master unit with a monitor detects that power is turned on, a bit error counter 205 which receives the pseudo-random pattern signal and counts error bits on the transmission line when the power-source activation detector for the slave unit with a camera detects that power is turned on, CPUs 103, 202 which variably control the transmission speed of the transmission line according to the error rate of the bits counted by the bit error counter, and transmission speed varying circuits 104, 203 which vary the transmission speed of the transmission line by the control of the CPUs.