A partial discharge sensor includes: a first ground (1) that constitutes a closed space (3) to surround a high-voltage wire (2); and a cylindrical branch pipe (4) disposed at the first ground (1) to protrude to an outer space to house with a lid (8) a first metal container (6) having at least one hole through which gas can pass, and having a slit (12) formed on the side to the space (3); at one end of a coaxial wire (7) disposed in the container (6), an outer conductor (9) of the coaxial wire (7) is connected to the first metal container (6) at a portion near the center of the slit (12), and an inner conductor (10) of the coaxial wire (7) is connected to the first metal container (6) at an opposite portion to a position where the outer conductor (9) is connected across the slit (12).