1.55-μm-InGaAsP photonic crystal lasers operating in the monopole mode of a triangular lattice defect cavity are realized by room-temperature optical pumping with a low threshold of <0.5 mW. By analyzing rate equations and the light–light characteristic curve, we evaluate the spontaneous emission factor (β) of this laser to be larger than 0.1, an extremely high value for photonic crystal lasers. The achievement of this large β is attributed to the facts that the investigated monopole-mode laser operates at the peak of the spontaneous emission spectrum and it shows a single-mode operation in a wide spectral range. In addition, nonclamping of carrier density near threshold and the β>0.1 are confirmed by the investigation of carrier-density dependent wavelength blueshift of the lasing mode.