A surface emitting laser comprising: a lower reflector (110, 810, 1302), an active layer (120, 820, 1304), and an upper reflector (100, 800, 1306) that are arranged in that order; an air gap (130, 830) provided between the active layer (120, 820, 1304) and the upper reflector (100, 800, 1306); and a slab (140, 850, 1318) provided on an optical path of the air gap (130, 830) and having a refractive index that differs from a refractive index of the air gap (130, 830), wherein at least one of the upper reflector (100, 800, 1306) and the lower reflector (110, 810, 1302) is configured so as to be movable in an optical axis direction so that a wavelength of emitted light changes in accordance with a change in a position of the at least one of the upper reflector (100, 800, 1306) and the lower reflector (110, 810, 1302) in the optical axis direction, characterized in that an optical thickness of the slab (140, 850, 1318) in the optical axis direction is in a range of greater than 0 and smaller than 1/2 a center wavelength of the surface emitting laser, or is the sum of a value in this range and half an integral multiple of the center wavelength; and in the case where the position of the upper reflector (100, 800, 1306) is changed, a center of the slab (140, 850, 1318) in the optical axis direction is located between any antinode of a standing wave formed in the air gap (130, 830) and a node of the standing wave on an upper-reflector side of the antinode, such that an amount of light present in the slab is increased when the air gap length is increased, and is reduced when the air gap length is reduced, and in the case where the position of the lower reflector (110, 810, 1302) is changed, the center of the slab (140, 850, 1318) in the optical axis direction is located between any antinode of the standing wave formed in the air gap (130, 830) and a node of the standing wave on a lower-reflector side of the antinode, such that an amount of light present in the slab is increased when the air gap length is increased, and is reduced when the air gap length is reduced. a lower reflector (110, 810, 1302), an active layer (120, 820, 1304), and an upper reflector (100, 800, 1306) that are arranged in that order; an air gap (130, 830) provided between the active layer (120, 820, 1304) and the upper reflector (100, 800, 1306); and a slab (140, 850, 1318) provided on an optical path of the air gap (130, 830) and having a refractive index that differs from a refractive index of the air gap (130, 830), wherein at least one of the upper reflector (100, 800, 1306) and the lower reflector (110, 810, 1302) is configured so as to be movable in an optical axis direction so that a wavelength of emitted light changes in accordance with a change in a position of the at least one of the upper reflector (100, 800, 1306) and the lower reflector (110, 810, 1302) in the optical axis direction, characterized in that an optical thickness of the slab (140, 850, 1318) in the optical axis direction is in a range of greater than 0 and smaller than 1/2 a center wavelength of the surface emitting laser, or is the sum of a value in this range and half an integral multiple of the center wavelength; and in the case where the position of the upper reflector (100, 800, 1306) is changed, a center of the slab (140, 850, 1318) in the optical axis direction is located between any antinode of a standing wave formed in the air gap (130, 830) and a node of the standing wave on an upper-reflector side of the antinode, such that an amount of light present in the slab is increased when the air gap length is increased, and is reduced when the air gap length is reduced, and in the case where the position of the lower reflector (110, 810, 1302) is changed, the center of the slab (140, 850, 1318) in the optical axis direction is located between any antinode of the standing wave formed in the air gap (130, 830) and a node of the standing wave on a lower-reflector side of the antinode, such that an amount of light present in the slab is increased when the air gap length is increased, and is reduced when the air gap length is reduced.