We experimentally demonstrate a thermal emitter with a narrow-bandwidth and low background emission, operating in the mid-wavelength infrared (MWIR) range, based on a combination of inter-subband transitions in GaN/AlGaN multiple quantum wells and optical resonance in a photonic crystal slab. The fabricated device exhibits single-peak narrowband thermal emission with a Q factor of 93 at a wavelength of 4.0 mu m. Stable operation at high temperatures over 700 degrees C has been demonstrated owing to the good thermal stability of GaN/AlGaN, which enables the generation of strong peak emission intensity as high as 93 mW/lm/sr/cm(2). Such a narrow-band and low-background emitter in the MWIR range has been difficult to realize by metal or heavily doped semiconductor-based emitters due to the broadband emission characteristics of the materials and by GaAs/AlGaAsbased emitters due to the thermal instability of the materials. Our device can be applied to various MWIR applications including CO2 and NOx gas sensing systems.