To provide a surface treatment device capable of restraining the generation of heat in a magnetic field generating part even if machining for a machining object is performed for a long time to stably perform machining for the machining object over a long period of time.

This surface treatment device 1 includes a storing vessel 9, an electromagnetic coil 8 and a cooling part 11. The storing vessel 9 is formed cylindrical to store both magnetic abrasive grains 65 and the machining object 2. The electromagnetic coil 8 generates a rotary magnetic field in the storing vessel 9, thereby revolving and rotating the magnetic abrasive grains to collide with the outer surface of the machining object 2. A space is formed between the inner peripheral surface of the electromagnetic coil 8 and the outer peripheral surface of the storing vessel 9. The cooling part 11 guides a pressurized gas from a pressurized gas supply source 67 to the space using a nozzle 69 to cool the electromagnetic coil 8.