A projection type image display apparatus has a light source unit (11) that includes an arc tube (110) enclosed by a reflector (111) for reflecting the light emitted from the arc tube in one direction through its opening (111a). The opening (111a) of the reflector (111) of the light source unit (11) is covered with a transparent member (112). Formed in one side of the reflector (111), adjacent the opening is an air inlet port (113), while the side facing the air inlet port (113) is closed (i.e. has no port). Formed in the remaining two sides of the reflector (111), and adjacent the opening (111a), are air outlet ports (114) one for each side. This arrangement causes the cooling air to be uniformly circulated in the entire reflector (111) of the light source unit (11) without stagnation to regulate the temperature of the arc tube (110) in an easy and costless manner, and enables installation of the light source unit (11) at any angular position, free of angular limitation.