PURPOSE: To prevent a sudden rise in pressure so as to prevent temperature rise and thermal fatigue resulting therefrom by taking gas accumulator pressure into a charge control portion, and gradually building up pressure to a value obtained by multiplying of the accumulator pressure by a predetermined constant, and performing charging to final pressure.

CONSTITUTION: A memory portion and an arithmetic portion 11 both of which constitute a charge control portion first take in the pressure (a) of an air accumulator 13 and determine whether nor not the pressure (a) has reached a maximum charging pressure and, when the pressure (a) has reached the maximum charging pressure, hold the pressure until gas temperature becomes a specified value. If the pressure (a) has not yet reached the maximum charging pressure, an optimum pressurization constant at that time is calculated and a value obtained by multiplying of the constant by the pressure of the air accumulator 13 is set as pressure (c) and then charging pressure (d) is taken in and is compared with the pressure (c). On the basis of the result of this comparison a control signal generating portion 12 sends a drive signal to a charging pressure regulating valve if (c)>(d), and applies a pulse for building up pressure. When (c)≤(d), application of the pulse is stopped and the pressurization constant is lowered by one rank and when a final charging pressure is reached a holding process is performed, while when the final charging pressure is not reached the above process is repeated for gradual charging.