PURPOSE: To obtain an economic and stable circuit by composing the circuit of a variable phase-shifting circuit and high-speed logic element.

CONSTITUTION: By pulse width shaping circuit 101, NRZ (Non-Return-to-Zero) signal 1 is shaped to a constant pulse width. Shaped NR2 signal 2 is applied to detecting circuit 201 so as to detect its high level and low level signals, thereby generating control circuit 7. On the basis of signal 7 from circuit 201, selecting circuit 202 selects fixed clock pulse 5 or variable clock pulse 6 outputted by variable phase-shifting circuit 204 and sends out clock pulse 8 which controls the pulse width of signal 2. Pulse-width shaping circuit 203 shapes the pulse width of signal with pulse 8 to generate completely controlled NRZ signal 4. Thus, circuit 204 provides economic and stable pulse-width control.