A maillard reaction inhibitor comprising, as the principal component, an eight-structural polymer of 3-oxygermylpropionic acid represented by the following stereostructure (I): ##STR1## wherein R represents --CH.sub.2 CH.sub.2 COOH, and m is a weight average polymerization degree calculated on the basis of the weight average molecular weight of propagermanium propyl ester and is 137.+-.84 (137 on average with a standard deviation of .+-.3.sigma.); and having the following minimum constitutional unit:(O.sub.1/2).sub.3 GeCH.sub.2 CH.sub.2 COOHand the following empirical formula:C.sub.6 H.sub.10 Ge.sub.2 O.sub.7,a method for inhibiting the Maillard reaction, and a method for preventing and treating diseases caused by Maillard reaction such as diabetes and concurrent diseases thereof.