Provided is a strainer capable of removing bubbles contained in a liquid and preventing the removed bubbles from mixing with the liquid again. The strainer includes a second filtration section having a substantially cylindrical shape with both ends connected, formed by folding a thin plate-shaped filter medium in a pleated shape, which is provided inside a first filtration section having a substantially cylindrical shape with both ends connected, formed by folding the thin plate-shaped filter medium in a pleated shape, without coming into contact with the first filtration section. An outer cylinder having a substantially cylindrical shape having a height higher than that of each of the first filtration section and the second filtration section is also provided outside the first filtration section, without coming into contact with the first filtration section. In addition, an upper plate, having a substantially discoid shape covering an upper side of each of the first filtration section, the second filtration section, and the outer cylinder, is formed with a plurality of first air vent holes between the outer cylinder and the first filtration section and formed with a plurality of second air vent holes between the first filtration section and the second filtration section, in a plan view.