A modified whey protein having improved keeping qualities and comprising a partly heat-denatured whey protein and a casein protein optionally together with an undenatured whey protein. The partly heat-denatured whey protein can be prepared by heating a whey protein, the protein concentration and pH value of which have been adjusted respectively to 0.5 to 15 % by weight and 6 to 8, at 55 to 120 DEG C for 1 to 120 minutes. The process for producing the modified protein comprises adding a casein protein to the partly heat-denatured whey protein or adding a casein protein to an undenatured whey protein to partly denature the same by heating. The modified whey protein having improved keeping qualities can provide a gel which has a good water retention and a high elasticity and is useful as a food material.