When an electropneumatic valve positioner is surrounded by combustible gas, the electrical control circuit should be located far from the actuator of the valve by connecting them using a long pneumatic tube to provide the greatest degree of safety. Then, if the length of the pneumatic tube exceeds a certain length, the actuator will occur hunting oscillations due to the time lag of the pneumatic pressure signals. The magnitude of this time lag is in accordance with the tube length, tube diameter and capacity of the actuator. This report investigates the step and frequency responses to define such a time lag theoretically and compares it with experiments carried out in foreign countries together with a discussion on it. As the result, the theoretical results of the step response agreed approximately with the experimental ones. The frequency response also agreed with them except for the phase lag in the case of the long tube. The studies on the valve positioner are being carried out using these results.