The present invention aims to provide a pattern dimension measurement method for accurately measuring an amount of shrinkage of a pattern that shrinks and an original dimension value before the shrinkage and a charged particle beam apparatus.In order to attain the above-mentioned object, there are proposed a pattern dimension measurement method and a charged particle beam apparatus that are characterized by: forming a thin film on a sample including the pattern after carrying out beam scanning onto a first portion of the pattern; acquiring a first measurement value by scanning a beam onto a region corresponding to the first portion on which the thin film is formed; acquiring a second measurement value by scanning a beam onto a second portion that has identical dimensions as those of the first portion on design data; and finding the amount of shrinkage of the pattern based on subtraction processing of subtracting the first measurement value from the second measurement value.