The present invention provides an ethylene-vinyl alcohol based copolymer (EVOH) having fundamentally improved thermal stability. The EVOH is characterized in that the proportion of ethylene units (III) is from 20 to 60 mole % with respect to the total (III+IV+V) of ethylene units (III), vinyl alcohol units (IV), and vinyl ester units (V), and the proportion of the total (I+II) of carboxylic acids units (I) and lactone ring units (II) in copolymer terminals with respect to the total (III+IV+V) is 0.12 mole % or less. The EVOH can be obtained by, for example, contacting a reducing agent with an ethylene-vinyl ester based copolymer before saponification and/or EVOH after saponification.