Abstract: The present invention has an objective to provide a projection display device of a U-shape structure with an illumination optical system folded by a mirror, which reduces heat load without using a mirror made of heat-resistant glass to fold light from a light source. A projection display device that folds light from a light source 1 composed of a lamp 1a and a parabolic metallic reflector 1b by a cold mirror 2 to be applied to light modulating elements 10B, 10G, 10R through fly-eye integrators 3, 4, thereby enlarging and projecting the light modulated by the light modulating element through a projection lens 20, wherein a cold mirror 2 is cooled by cooling air from a fan 32, ultraviolet and infrared rays transmitted through the cold mirror 2 are emitted out of a cabinet 30, so that retroreflection of ultraviolet and infrared rays to a lamp 1a is prevented.