A brake chamber includes a rod member that makes reciprocating motion within a cylinder through a pressure of compressed air or an urging force of a spring, thereby causing a pushrod for activating a brake device of a vehicle to proceed into the brake device. The brake chamber further includes a diaphragm that deforms in response to supply of the compressed air to push the rod member, and a housing that forms an accommodation space for the diaphragm. The housing is connected to the cylinder with a first end of the cylinder being inserted in the housing. The brake chamber further includes a boot member connected to the cylinder and the rod member. The boot member includes a sealing portion to prevent entry of foreign matter into a connection portion between the housing and the cylinder by contacting an inner circumferential surface of the housing.