An amplitude adjusting circuit comprises a first current mirror where a variable current of a variable current source is copied into each of 1st-3rd transistors; a second current mirror where the variable current is copied into each of 11th-13th transistors; a third current mirror having 6th-7th transistors where a current through the 2nd transistor copied from the variable current flows through the 6th transistor; a fourth current mirror having 8th-9th transistors where a current through the 12th transistor copied from the variable current flows through the 8th transistor; an inverter that has 1st-2nd conductivity type transistors and produces an output signal corresponding to a current level of the 7th or 9th transistor; a fifth current mirror having 15th-14th transistors where a current through the 14th transistor copied from the 15th transistor's becomes a current sourced by the 7th transistor; and a sixth current mirror having 5th-4th transistors where a current through the 4th transistor copied from the 5th transistor's becomes a current sunk by the 9th transistor.