A monitoring apparatus (1, 1a) for a gas-insulated switching apparatus, the monitoring apparatus (1, 1a) comprising: a monitoring panel that is configured to acquire monitoring information of the gas-insulated switching apparatus from a monitoring sensor; and an antenna unit (3, 3a) that is connected to the monitoring panel and capable of transmitting and receiving the monitoring information by wireless communication, characterized in that: the antenna unit (3, 3a) comprises: a first metal plate that is placed on the monitoring panel horizontally and grounded; a monopole antenna that is placed on the first metal plate; a second metal plate that is placed above and in parallel with the first metal plate and faces the first metal plate; and a plurality of metal posts that is placed so as to surround the monopole antenna and couples the first metal plate to the second metal plate, wherein a distance between the first metal plate and the second metal plate has a length that is a half of a wavelength of a radio wave used in the wireless communication, and the plurality of metal posts are arranged in such a manner that each distance between neighboring posts has a length that is a half of the wavelength. a monitoring panel that is configured to acquire monitoring information of the gas-insulated switching apparatus from a monitoring sensor; and an antenna unit (3, 3a) that is connected to the monitoring panel and capable of transmitting and receiving the monitoring information by wireless communication, characterized in that: the antenna unit (3, 3a) comprises: a first metal plate that is placed on the monitoring panel horizontally and grounded; a monopole antenna that is placed on the first metal plate; a second metal plate that is placed above and in parallel with the first metal plate and faces the first metal plate; and a plurality of metal posts that is placed so as to surround the monopole antenna and couples the first metal plate to the second metal plate, wherein a distance between the first metal plate and the second metal plate has a length that is a half of a wavelength of a radio wave used in the wireless communication, and the plurality of metal posts are arranged in such a manner that each distance between neighboring posts has a length that is a half of the wavelength. the antenna unit (3, 3a) comprises: a first metal plate that is placed on the monitoring panel horizontally and grounded; a monopole antenna that is placed on the first metal plate; a second metal plate that is placed above and in parallel with the first metal plate and faces the first metal plate; and a plurality of metal posts that is placed so as to surround the monopole antenna and couples the first metal plate to the second metal plate, wherein a first metal plate that is placed on the monitoring panel horizontally and grounded; a monopole antenna that is placed on the first metal plate; a second metal plate that is placed above and in parallel with the first metal plate and faces the first metal plate; and a plurality of metal posts that is placed so as to surround the monopole antenna and couples the first metal plate to the second metal plate, wherein a distance between the first metal plate and the second metal plate has a length that is a half of a wavelength of a radio wave used in the wireless communication, and the plurality of metal posts are arranged in such a manner that each distance between neighboring posts has a length that is a half of the wavelength.