A projection display unit (1) includes a projection optical system (10A), a polarization separation device (15), and a detection optical system (10B). The projection optical system includes an illuminator (11), a projection lens (16), and a light valve (12) that modulates illumination light supplied from the illuminator on the basis of an image signal, and outputs the modulated illumination light toward the projection lens. The polarization separation device (15) is disposed between the light valve and the projection lens. The polarization separation device separates entering light into a first polarized component and a second polarized component, and outputs the first polarized component and the second polarized component in respective directions that are different from each other. The detection optical system includes an imaging device (13) and a reduction optical system (14). The imaging device is disposed in a position that is optically conjugate with a position of the light valve. The reduction optical system is disposed between the imaging device and the polarization separation device. The imaging device receives, via the projection lens and the polarization separation device, light based on detection invisible light. A transmittance adjuster is provided between the polarization separation device and the imaging device. The transmittance adjuster adjusts transmittance of at least part of a bundle of passing light rays derived from the invisible light.