A high pressure mercury lamp in which formation and spreading of milky opacification in the fused silica glass forming the discharge vessel can be advantageously prevented, and thus a rapid decrease of screen illuminance prevented, when it is used as the light source of a liquid crystal projector and the like by, in a high pressure mercury lamp in which a discharge vessel of fused silica glass contains a pair of opposed tungsten electrodes, mercury in an amount that is at least equal to 0.16 mg/mm.sup.3, rare gas, and at least one halogen, and in which the wall load is at least equal to 0.8 W/mm.sup.2, the amount of mercury added being fixed in a range from 2.times.10.sup.-4 to 7.times.10.sup.-3 .mu.mole/mm.sup.3, and/or the at least one halogen being in the form of a carbonless halogen compound, and/or the average OH radical concentration in an area of a wall of the discharge vessel at a depth of 0.2 mm from an inner surface of the wall of the discharge vessel being at most 20 ppm.