An internally grooved heat transfer tube for a cross fin tube type heat exchanger of a refrigerating air-conditioning water supply apparatus using a high-pressure refrigerant, wherein an intra-tubular heat transfer rate is improved while maintaining a sufficient strength for pressure resistance.A heat transfer tube formed of copper or copper alloy has internal fins between internal grooves. In the tube, t/D is not smaller than 0.041 and not greater than 0.146, d.sup.2/A is not smaller than 0.75 and not greater than 1.5, where D [mm] is an outside diameter of the tube, t [mm] is a groove bottom thickness, d [mm] is a depth of each groove, and A [mm.sup.2] is a cross sectional area of each groove. N/Di is not smaller than 8 and not greater than 24 where N is a number of grooves, and Di is a maximum inside diameter corresponding to an inside diameter of the tube.

 
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