In a signal transmission system of a key telephone system of the type comprising a main device connected to office lines and having an input/output port for transmitting and receiving signals, and a plurality of key telephone sets connected in parallel to the main device through a pair of signal lines for transmitting and receiving various control signals between the main device and the key telephone sets, wherein each of the control signals is constituted by a pulse width modulated time series signal of a predetermined number of bits, respective control signals are inputted and outputted in parallel to and from a bit input/output of the input/output port and wherein transmission and reception of the control signals between the main device and respective key telephone sets are simultaneously performed by parallel input/output operations of the calculating processing unit.