PURPOSE: To increase energy efficiency and improve durability of components such as vaporization cylinder and backpressure valve in a vapor phase combined heating pressure reducing system.

CONSTITUTION: A backpressure valve 4a is provided in a liquid phase line 2 and set in such a way that it is opened at the predetermined pressure or less. When pressure in a liquefied gas container 1 is higher than the set pressure of the backpressure valve 4a, gas is pressure-reduced and supplied from a vapor phase line 3 by a vapor phase line pressure regulating valve 9a. When pressure in the container 1 is lower than the set pressure of the backpressure valve 4a, gas is vaporized by a vaporization cylinder 5, gas pressure is reduced by a liquid phase line pressure regulating valve 8, and gas can be also supplied by the liquid phase line 2. When inlet pressure of the vapor phase line pressure regulating valve 9a is below the set pressure of the back pressure valve 4a, the vapor phase line pressure regulating valve 9a is controled to set outlet pressure of the vapor phase line pressure line regulating valve 9a to higher pressure than the set pressure of liquid phase pressure regulating valve 8 so that the vapor phase line pressure regulating valve 9a is operated at all times.