Abstract: A warning apparatus comprising a plurality of warning mat units interconnected and arranged side by side in a such manner as to surround a dangerous area, in which the warning mat units each comprise a base plate, a flexible cover sheet secured to an upper side of the base plate, the cover sheet being provided with a plurality of upwardly expanded shells having an inside hollow room and air circulation apertures, and a warning sound generator accommodated in the inside room of the shell, in which the warning sound generator comprises a closed elastic tube fastened on the base plate and a whistle affixed to a wall aperture of the closed elastic tube so as to give forth sound by the effect of air flow passing between an inside and an outside of the closed elastic tube.