A spatial light modulator has a plurality of mirror elements each of which is controllable into a first state in which the mirror element reflects incident light with a change in a phase thereof by a first phase and a second state in which the mirror element reflects the incident light with a change in the phase thereof by a second phase 180° different from the first phase; and a boundary portion arranged between the mirror elements, which changes the phase of the incident light by a third phase substantially (90°+k·180°) (where k is an integer) different from the first phase. In projecting a pattern with the use of the spatial light modulator, an error caused in the pattern can be reduced even if the light quantity of light passing a gap region between the optical elements in the spatial light modulator is large.