PURPOSE: To provide a simply constituted ultraprecise pressure adjustment electromagnet body at a low cost by releasing pressure from an orifice by the control of a permanent magnet position to the orifice and maintaining the pressure inside a tank at a prescribed value.

CONSTITUTION: An electromagnet 10 is provided with an inner side magnetic pole piece 10c between outer side magnetic pole pieces 10a and 10b, the magnetic pole pieces are coupled through a base element 10d and an E-shaped structure is formed. A coil 11 is provided on the inner side magnetic pole piece 10c, a permanent magnet 12 is freely movably attached to the inner side magnetic pole piece 10c and the orifice 15 is formed at the extension part 14 of the magnetic pole piece 10b. The polarity of the magnetic pole piece and magnetic force to act on the permanent magnet 12 are determined by the polarity and an amount of a current flowing to the coil 11. When tank pressure is lowered for instance, since a servo mechanism changes the current flowing to the coil 11 and the direction thereof and the permanent magnet 12 approaches the orifice 15, a discharge speed is lowered and the tank pressure is raised. Thus, the tank pressure is precisely controlled with simple constitution.