Abstract: [Problem] To reduce the size, lower the cost and increase and flatten the thrust of an electromagnetic actuator in which a plunger moves reciprocally. [Solution] The electromagnetic actuator comprises a yoke 130, a coil 180 arranged around the yoke, an armature 120 slidably arranged inside the yoke and integrally formed with a plunger 110, and a return spring 150 for returning the armature to a rest position, wherein the yoke 130 has, in a predetermined position on the axial direction L, a thickness-reduced annular gap groove 133 over part of an outer peripheral face 131, the thickness-reduced portion having a trapezoidal cross section widening outwards.