An apparatus warns of differential pressure during the opening of a pressure-loaded door that closes an opening in an aircraft fuselage. An air guide passage extends from the side with higher pressure to the side with lower pressure. The air guide passage is closeable by a valve that is controllable with a control lever operatively Connected to a door opening mechanism. When there is a differential pressure between the two sides, upon opening of the valve the apparatus produces an acoustic signal as a differential pressure indication that is independent of a supply system and serves as a warning signal for the operator of the aircraft door.