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We have successfully fabricated InAs quantum dots (QDs) embedded in a line-defect waveguide in an air-bridge type GaAs photonic-crystal slab (PCS) and observed laser action from optical-pumping. This lasing is found to occur without any optical cavity, such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the triple-lines defect mode enables us to specify the lasing wavelength as that at the band edge. From this fact, it follows that the distributed feedback mechanism at the band edge with an infinitely small group velocity is responsible for the present lasing.