We have carried out electron tunnelling spectroscopy on single crystals with the use of a scanning tunnelling microscope at 4.2 K. A conductance peak structure was observed near zero bias voltage for the tunnelling along the Cu-O bonding direction. A clear superconducting gap was observed for the tunnelling along the Cu-Cu direction. Curves along the Cu-O and Cu-Cu directions are consistent with the -wave pairing symmetry which has line-nodes along the Cu-Cu direction, taking into account the periodic arrangement of surface oxygen atoms.