The present invention relates to a device for detecting a partial discharge for a power transformer which detects an electromagnetic signal occurring due to faulty insulation. The device includes an antenna unit receiving electromagnetic waves, an insulator including the antenna unit, a metallic air-tight unit that seals a connector connected to the insulator and connecting a coaxial cable, and the coaxial cable exposed to the outside of the metallic air-tight unit. Thus, it is possible to enhance broadband properties through an internal conductor of a drain valve.