PURPOSE: To improve the characteristic of a transmitter by preventing spurious radiation caused by a multiple wave from a multiplier, in the frequency converter multiplying an output of an oscillator to obtain a local oscillation signal.

CONSTITUTION: An output of an oscillator 2 is multiplied by (n) at an n- multiplier 3 to obtain a local oscillation signal and it is given to a mixer 7 to frequency-convert a signal from an IF input terminal 9 and the frequency converted signal is outputted from an RF output terminal 10 as the frequency converter. A 1st filter 5 employed in the frequency converter blocks an (n+1)- multiple wave or an (n-1)-multiple wave thereby preventing the waves from being directly outputted. Furthermore, a 2nd filter 4 is employed to block a (2n+1)-multiple wave or a (2n-1)-multiple wave being a caused to a neighbouring wave generated by the mixer.