We have designed, fabricated, and characterized an InP photonic crystal slab structure that supports a cavity-confined slow-light mode, i.e. a bandgap-confined valence band-edge mode. Three dimensional finite difference in time domain calculations predict that this type of structure can support electromagnetic modes with large quality factors and small mode volumes. Moreover these modes are robust with respect to fabrication imperfections. In this paper, we demonstrate room-temperature laser operation at 1,5 μm of a cavity-confined slow-light mode under pulsed excitation. The gain medium is a single layer of InAs/lnP quantum dots. An effective peak pump power threshold of 80 μW is reported.