PURPOSE: To avoid making a mass spectro meter large-sized without enlarging the breadth of a magnetic pole by forming uniform electric field at a gap even in use for the passage of beam having a large sectional area.

CONSTITUTION: Auxiliary magnetic poles 11 to 14 as a set are laid at both sides of a gap 5 as a beam passage between main magnetic poles 2 and 3. A magnetic line of force La between the auxiliary magnetic poles 11 to 14 is kept in the same direction as a magnetic line of force L between the main magnetic poles 2 and 3. As a result, both magnetic lines of force L and La are repulsive to each other, thereby restraining the magnetic line of force L between the main magnetic poles 2 and 3 to swell outside. According to this construction, uniform magnetic field can be formed at the gap 5.