A quantum cascade laser element comprising a superlattice formed by a crystal of a nitride semiconductor, wherein the superlattice includes a plurality of unit structures, wherein each unit structure is formed to consist of a first barrier layer, a first well layer, a second barrier layer, a second well layer, a third barrier layer, and a third well layer disposed in this order by repeatedly stacking a barrier layer and a well layer respectively having high and low potentials with respect to conduction-band electrons, wherein, in each unit structure, an energy level structure for electrons under a bias electric field in a stacking direction due to external voltage has: a transport level that has a significant probability of finding an electron in the first well layer; an upper lasing level that has a significant probability of finding an electron in the second well layer; a lower lasing level that has a significant probability of finding an electron In the third well layer; and a depopulation level that has a significant probability of finding an electron in the third well layer, wherein the upper lasing level has an energy value lower than the transport level by at least an energy value of a longitudinal-optical phonon of the crystal of the nitride semiconductor making the superlattice, and wherein the depopulation level has an energy value lower than an energy value of the lower lasing level by at least an energy value of the LO-phonon, the energy value of the LO-phonon of the nitride semiconductor making the superlattice being greater than a photon energy for an electromagnetic wave to be emitted by stimulated emission from an electron that makes a transition from the upper lasing level to the lower lasing level under the bias electric field. wherein the superlattice includes a plurality of unit structures, wherein each unit structure is formed to consist of a first barrier layer, a first well layer, a second barrier layer, a second well layer, a third barrier layer, and a third well layer disposed in this order by repeatedly stacking a barrier layer and a well layer respectively having high and low potentials with respect to conduction-band electrons, wherein, in each unit structure, an energy level structure for electrons under a bias electric field in a stacking direction due to external voltage has: a transport level that has a significant probability of finding an electron in the first well layer; an upper lasing level that has a significant probability of finding an electron in the second well layer; a lower lasing level that has a significant probability of finding an electron In the third well layer; and a depopulation level that has a significant probability of finding an electron in the third well layer, a transport level that has a significant probability of finding an electron in the first well layer; an upper lasing level that has a significant probability of finding an electron in the second well layer; a lower lasing level that has a significant probability of finding an electron In the third well layer; and a depopulation level that has a significant probability of finding an electron in the third well layer, wherein the upper lasing level has an energy value lower than the transport level by at least an energy value of a longitudinal-optical phonon of the crystal of the nitride semiconductor making the superlattice, and wherein the depopulation level has an energy value lower than an energy value of the lower lasing level by at least an energy value of the LO-phonon, the energy value of the LO-phonon of the nitride semiconductor making the superlattice being greater than a photon energy for an electromagnetic wave to be emitted by stimulated emission from an electron that makes a transition from the upper lasing level to the lower lasing level under the bias electric field.