PURPOSE: To display remaining speech enable period of time at a public telephone set during speaking.

CONSTITUTION: The lapse of time per one frequency is calculated by measuring a period of time from a point of the detection of an opposite party response signal by an opposite party response detection part 4 to the detection of a first charging pulse by a charging pulse detection part 3, and the measured result is outputted to a multiplication part 13. On the other hand, a remaining frequency counter 5 decreases frequencies one by one at every time of charging pulse detection by inputting the information of an inserted coin 1 or telephone card 2 and the output data of the charging pulse detection part 3 and the opposite party response detection part 4. Each time the remaining frequency value is changed, the counted result is outputted to the multiplier 13. The multiplier 13 calculates the remaining speech enable period of time at the time of charging pulse detection from the outputs of a remaining frequency counter part 5 and one-frequency time measuring part 12. A subtracting counter part 14 subtracts the calculated value of the multiplier part at every lapse of the unit time shorter than a charging pulse detection interval and displays the result at a speech enable time display part 15.