Here we investigate a line-defect-waveguide laser in a two-dimensional photonic-crystal slab. The change δλ in lasing wavelength for a change δW in waveguide width is found to be extremely large: δλ/δW=2. It is two orders of magnitude greater than that for a conventional distributed-feedback laser. An investigation of the electromagnetic-field distribution at the waveguide-mode edge is made, giving an indication of the cavity-mode structure for lasing oscillation. Fourier transform analysis reveals that the extremely large tunability originates from the “zigzag” mode behavior of the line-defect waveguide in the photonic crystal, due to two-dimensional distributed feedback and reflection.