1. A pattern dimension measuring apparatus comprising: unit for irradiating an electron beam onto a sample in a scanning manner; unit for acquiring image data of a pattern according to the quantity of electrons emitted, by irradiation of the electron beam, from a surface of the sample where the pattern is formed; and area measuring unit for dividing the pattern into a plurality of partial patterns, calculating the areas of the partial patterns, and calculating the area of the pattern by summing up the areas of the partial patterns.