A single balanced mixer has a high degree of isolation between an IF signal and an RF signal and a high conversion efficiency. The single balanced mixer includes means for producing two local signals of same amplitude and opposite phase, a pair of mixing elements each receiving a corresponding one of the two local signals, a pair of strip lines for transmitting the input RF signal to the mixing elements. One end of each of the strip lines is connected to one another at a point where the RF signal is supplied and other end of each of the strip lines is connected to the corresponding mixing element. A length of each of the strip lines is one fourth of a wave length of the IF signal.