We investigated lasing characteristics of resonance modes in a thin dielectric coated square cavity with round corners. After coating dye-doped PMMA on a square capillary, a thin polystyrene layer with a thickness of about 0.5 micron was overcoated. Lasing peaks corresponding to ring-type whispering gallery modes were observed in air environment. When the capillary was inserted into glycerine whose refractive index was slightly smaller than PMMA, the mode spacing of lasing peaks was severely reduced. We confirmed this reduced lasing peaks corresponded to waveguide-type modes distributing on the polystyrene layer and the laser gain was originated from evanescent-wave coupling mechanism.