A yoke is provided with a notch, notched from two points P1 and P2 on a circumference forming a sleeve-side insertion hole toward the outside of the circumference, the notch having a minimum distance between the points P1 and P2 as a width W and the distance from a straight line linking the points P1 and P2 in a direction orthogonal to the straight line as a length L. The width W of the notch is set on the basis of a target attraction force for attracting a plunger, the attraction force increasing with an increase in the width W, and the length L of the notch is greater than or equal to a prescribed length Ld where the attraction force increasing with an increase in the length L becomes constant at the target attraction force in the set width W irrespective of an increase in the length L.