A starter comprising: a motor operative to generate a rotational force which is delivered to a ring gear; an output shaft operative to rotate upon receipt of the rotational force of the motor; a pinion gear slidably disposed on the output shaft to be axially movable; an electromagnetic switch having a switch coil forming a first electromagnet to generate a first attraction force when turned on for closing a main contact disposed in a motor energizing circuit of the motor; a solenoid having an excitation coil forming a second electromagnet to generate a second attraction force when turned on for moving the pinion gear toward the ring gear of the engine; and a unitary structure supporting the electromagnetic switch and the solenoid in series on a coaxial relationship characterized in that: the electromagnetic switch and the solenoid include a switch case, forming a part of a first magnetic circuit of the switch coil at an outer circumferential periphery thereof, and a solenoid case, forming a part of a second magnetic circuit of the excitation coil at an outer circumferential periphery thereof, the switch case and the solenoid case being unitarily formed to be contiguous with each other in an axial direction, wherein: the switch case and the solenoid case take the form of a double-layer structure including an inner yoke, placed in a radially inward area, and an outer yoke placed in a radially outward area; wherein the inner yoke has a cylindrical configuration with both ends being opened, and the outer yoke has a bottomed cylindrical configuration with one end formed in an annular bottom wall. a motor operative to generate a rotational force which is delivered to a ring gear; an output shaft operative to rotate upon receipt of the rotational force of the motor; a pinion gear slidably disposed on the output shaft to be axially movable; an electromagnetic switch having a switch coil forming a first electromagnet to generate a first attraction force when turned on for closing a main contact disposed in a motor energizing circuit of the motor; a solenoid having an excitation coil forming a second electromagnet to generate a second attraction force when turned on for moving the pinion gear toward the ring gear of the engine; and a unitary structure supporting the electromagnetic switch and the solenoid in series on a coaxial relationship characterized in that: the electromagnetic switch and the solenoid include a switch case, forming a part of a first magnetic circuit of the switch coil at an outer circumferential periphery thereof, and a solenoid case, forming a part of a second magnetic circuit of the excitation coil at an outer circumferential periphery thereof, the switch case and the solenoid case being unitarily formed to be contiguous with each other in an axial direction, wherein: the switch case and the solenoid case take the form of a double-layer structure including an inner yoke, placed in a radially inward area, and an outer yoke placed in a radially outward area; wherein the inner yoke has a cylindrical configuration with both ends being opened, and the outer yoke has a bottomed cylindrical configuration with one end formed in an annular bottom wall. the electromagnetic switch and the solenoid include a switch case, forming a part of a first magnetic circuit of the switch coil at an outer circumferential periphery thereof, and a solenoid case, forming a part of a second magnetic circuit of the excitation coil at an outer circumferential periphery thereof, the switch case and the solenoid case being unitarily formed to be contiguous with each other in an axial direction, wherein: the switch case and the solenoid case take the form of a double-layer structure including an inner yoke, placed in a radially inward area, and an outer yoke placed in a radially outward area; wherein the inner yoke has a cylindrical configuration with both ends being opened, and the outer yoke has a bottomed cylindrical configuration with one end formed in an annular bottom wall. the switch case and the solenoid case take the form of a double-layer structure including an inner yoke, placed in a radially inward area, and an outer yoke placed in a radially outward area; wherein the inner yoke has a cylindrical configuration with both ends being opened, and the outer yoke has a bottomed cylindrical configuration with one end formed in an annular bottom wall.