A key telephone system including (a) at least three telephones, and (b) a key telephone unit connected to the three telephones through at least three communication lines including at least one public switched line, the key telephone unit including (b1) at least three connection ports to each of which each of the telephones is connected through the communication lines, each of the connection ports having a predetermined number, (b2) a first unit which informs a first telephone of numbers of vacant connection ports and a number of the connection port to which a second telephone making a call to the first telephone is connected, when the first telephone makes a call, and (b3) a controller which connects the first telephone to one of the second telephone and a third telephone, based on the numbers of vacant connection ports and the number of the connection port to which the second telephone is connected.