To obtain a resin composition which has a good moldability and can give a product exhibiting an excellent elongation at break and mechanical properties without any complicated step such as stretching and having a good biodegradability by mixing a saturated aliphatic polyester having a melt viscosity in a specified range with starch and a polyethylene glycol having a number- average molecular weight in a specified range.

A saturated aliphatic polyester (a) having a melt viscosity of 1,000-15,000P at 200°C is mixed with starch (b) and a polyethylene glycol (c) having a number-average molecular weight of 100-20,000 in such amounts that the ratio of component (a) to component (b) is 95/5 to 60/40 by weight, and 1-200 pts.wt. component (b) is present per 100 pts.wt. total of components (a) and (c). It is desirable that the saturated aliphatic polyester is one having a linear structure, because it can give a biodegradable resin composition having a higher rate of biodegradation. With a view to the inhibition of hydrolysis during melt kneading, the starch is desirably one in the form of a powder having a mean particle diameter of 50μm or below and predried at 150°C for about 5hr to a water content of 1wt.% or below.