A composite-optical-system unit (40) includes a plurality of polarization beam splitters (400A, 400B, and 400C), a pair of spacer plates (420 or the like), and an optical element (411). Each of the pair of spacer plates (420) is arranged between the first polarization beam splitter (400A) and the second polarization beam splitter (400C) of the plurality of polarization beam splitters such that a first contact surface is fixed on an emission-side surface (403) of the first polarization beam splitter (400A) by surface contact, and a second contact surface is fixed on an incident-side surface (405a) of the second polarization beam splitter (400C) by surface contact. The optical element (411) is arranged between the pair of spacer plates (420).