In order to examine the role of supraglacial debris during ablation, a field experiment was carried out on a snowpatch beside the Rikha Samba Glacier, Hidden Valley. Black phyllite sand was scattered on nine test fields varying in thickness from about 0.5 cm to 8 cm. The thickness of the debris cover and the amount of surface ablation by melting of snow were measured carefully at 5 points in each test field. The results are as follows : (1) The ablation rate was accelerated under a thin debris layer and was retarded under a thick one as compared with that of a natural snow surface. The critical thickness of the debris layer was 1.6 cm. (2) The acceleration of ablation was greatest under a debris cover 0.5 cm thick.