This unit brake, which is equipped with a cylinder apparatus in which a clutch mechanism that switches between transmitting or cutting off a biasing force of a spring brake part is disposed, is capable of preventing a braking force due to the spring brake part from unintentionally decreasing due to a meshing section of the clutch mechanism disengaging, and is of a size that can replace an existing unit brake, and maintains the performance of bearings in the clutch mechanism even with long-term use. The clutch mechanism of the unit brake (100) comprises: a nut member (81) that rotatably screws on a spindle (50) that is positioned in an area that communicates with the atmosphere, and is movably supported in a direction opposite to braking; a clutch (84) that is disposed in a direction opposite to braking with respect to the nut member (81), and faces the nut member (81) in the vicinity of the spindle (50); a clutch box (82) that is formed in a cylinder shape, and houses the nut member (81) and clutch (84) on the inside; and bearings (83) that rotatably support the nut member (81) on the inside of the clutch box (82).