An electromagnetic wave applying apparatus according to the present invention has an electromagnetic wave source (13) such as an ultraviolet lamp or the like, a cylinder (15) surrounding the electromagnetic wave source, a liquid retention tank (16) disposed around the cylinder, and an inlet portion (17) for introducing a liquid overflow from the liquid retention tank as a thin film flowing down an inner wall surface of the cylinder, which is irradiated with an electromagnetic wave from the ultraviolet lamp (11). The electromagnetic wave applying apparatus also has swirling flow forming means for causing the liquid introduced from the inlet portion (17) onto the inner wall surface of the cylinder to flow as a swirling flow down the inner wall surface. The electromagnetic wave applying apparatus allows a large amount of liquid to be stably treated by the application of an electromagnetic wave without causing the electromagnetic wave source such as an ultraviolet lamp or the like to be contaminated.