PURPOSE: To reduce magnetic loss of a laminated core by a method wherein the surface treatment is applied to tightening plates or tightening pins or both of them at least on the parts where the plates and the pins touch each other and the electrical contact resistance is increased.

CONSTITUTION: A plunger, or a moving iron core 2 of an AC solenoid 1 is composed by laminating thin silicon steel plates 4 which are held and fixed between fastening plates 5 made of thicker steel plates by tightening pins 7. Electrical insulation films are formed on one ore both sides of the head portions of the pins 7 where the pins 7 touch the fastening plates 5 by surface treatment prior to the assembly. A core or a fixed core 3 is also composed by laminating thin silicon steel plates 4 which are held and fixed between fastening plates 6 by tightening pins 8. Electrical insulation films are also formed on the portions where the pins 8 touch the fastening plates 6 by surface treatment. With this constitution, the electrical contact resistance is increased and the short circuit loss is reduced, so that the power consumption is lowered.