A projection display unit includes a projection optical system, a polarization separation device, and a detection optical system. The projection optical system includes an illuminator, a projection lens, and a light valve that modulates illumination light supplied from the illuminator, and outputs the modulated illumination light toward 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 and a reduction optical system. The imaging device receives, via the projection lens and the polarization separation device, light based on detection invisible light. A transmittance adjuster provided between the polarization separation device and the imaging device adjusts transmittance of at least part of a bundle of passing light rays derived from the invisible light.