In an electromagnetic switch for an auxiliary-rotation starter according to the present invention, the magnetic bypass core is disposed at a place that is an air gap portion, in a magnetic circuit, through which the plunger moves due to magnetic attractive force produced by energizing the attraction coil and the holding coil and that is between the outer circumference of the plunger and the inner circumference of the bobbin, in such a way that one end thereof makes contact with one end of the case and the other end thereof faces the end of the core by the intermediary of an air gap.