A backpack for back-mounting a compressed-air cylinder, fastened to the backpack by a cylinder fastening belt, when a user wears harnesses and a waist belt. The backpack inlcudes a backboard (100) having support hooks (105) to support the compressed-air cylinder (50); a harness support (130) for the harnesses (30) coupled to the backboard (100) so as to rotate upwards and downwards around the center; a waist protector (120) coupled to the backboard (100) in the back of the harness support (130) so as to cover and protect the back of the user's waist; and a rotary unit for rotatably supporting both the harness support (130) and the waist protector (120) on the backboard (100). The waist protector and the harness support move separately from the backboard by the rotary unit, thus allowing the user to work without stressed by the waist belt or the harnesses.