PURPOSE: To make the rise of an output signal steep by processing and inputting an input signal via a photocoupler by an output circuit and a pullup circuit.

CONSTITUTION: An input signal IS to a transmission circuit 1A is photoelectric- converted by a photocoupler 4, inverted by an inverter 5 of an output circuit and when the level of the signal IS is at L, an output transistor(TR) 6 is turned on. On the other hand, the output of the photocoupler 4 is differentiated by a time constant circuit comprising a capacitor 10 and a resistor 11, a signal S2 being at an L level for a prescribed period t1 is outputted from an inverter 12 whose threshold level is set by a differentiation signal S1, a pullup TR 13 is turned on for the period t1 thereby improving unsharpened waveform of an output signal OS via the pullup resistor 14. Thus, the rise of the signal OS is made steep even if the number of reception terminal equipments is less thereby prolonging the distance of a transmission line 9.