To reduce magnetic resistance and to improve magnetic efficiency by constituting a magnetic frame by combining a plural number of magnetic frame bodies made of continuous upper, lower, left and right piece parts and using this magnetic frame.

An upper side part 38 and a lower side part 41 of each of magnetic frame bodies 35, 36 are closely engaged with an engaging groove 46 of a flange part 48 on the upper side of a bobbin 25 and an engaging groove 47 of a flange part 49 on the lower side of the bobbin 25. Thereafter, an upper ring is fitted on an outer periphery of a circular arc projected part 38c' of an upper side part 38 of each of the magnetic frame bodies 35, 36, and a lower ring is fitted on an outer periphery of a circular arc projected part 41c' of a lower side part 41 of each of the magnetic frame bodies 35, 36. Consequently, the first magnetic frame body 35 and the second magnetic frame body 36 are combined on an outer surface of a guide pipe and constitute a magnetic frame. Thereafter, a circular arc part 38c of the upper side part 38 of each of the magnetic frame bodies 35, 36 is arranged in the neighbourhood of a fixed iron core, and the lower side part 41 of each of the magnetic frame bodies 35, 36 is arranged in the neighbourhood of a movable iron core.