1. A two-dimensional photonic crystal laser, including: a two-dimensional photonic crystal made of a plate-shaped member provided with a periodic arrangement of many identically-shaped modified refractive index areas having a refractive index different from that of the plate-shaped member; and an active layer provided on one side of the two-dimensional photonic crystal, wherein: the modified refractive index areas are arranged at lattice points of a lattice with a same period at least in two directions; a shape of each modified refractive index area is such that a feedback strength is different with respect to directions of two primitive reciprocal lattice vectors of the lattice; the two-dimensional photonic crystal has a periodic structure of a supercell, as a unit, which contains a plurality of lattice points; and a sum of the feedback strengths by all modified refractive index areas in the supercell is identical in each direction of the two primitive reciprocal lattice vectors.