Disclosed herein is a full duplex transmission circuit including: a first internal input terminal receiving a signal to be transmitted; a second internal input terminal receiving a signal having an amplitude equal to ½ times the amplitude of the signal to be transmitted and having the same phase as the phase of the signal to be transmitted; an external input/output terminal; an internal output terminal; a first metal oxide semiconductor transistor; and the second metal oxide semiconductor transistor. A current generated by the current source as well as the sizes of the first and second metal oxide semiconductor transistors are set so that the transconductances of the first and second metal oxide semiconductor transistors become equal to 1/Z.