To specify the direction of the nodes in the superconducting gap, we measured the thermal conductivity K of quasi-2D heavy-fermion CeCoIn5 in a magnetic field rotating within the 2D planes. A clear fourfold symmetry Of K which is characteristic of a superconducting gap with nodes along the (+/-pi, +/-pi)-directions is resolved. The thermal conductivity also reveals a first order phase transition at H-c2. The results indicate that the symmetry most likely belongs to d(x2-y2), implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.