1. A pattern measurement apparatus comprising: a library creating unit which acquires a plurality of sectional shapes of a first pattern corresponding to a plurality of process parameters for creation of the pattern, respectively, uses the acquired plurality of sectional shapes to calculate predicted spectral waveforms which would be obtained when light is applied to the first pattern, and adds information on the corresponding process parameters to the calculated predicted spectral waveforms, respectively, to form a waveform library; a spectral waveform acquiring unit which actually acquires a spectral waveform of a second pattern as a measurement target, the second pattern being obtained by actually creating the first pattern by use of a process parameter set to obtain a desired shape; a process parameter calculating unit which performs waveform matching between the actually acquired spectral waveform and the predicted spectral waveforms to acquire matching scores for respective waveform matching, and calculates an optimum process parameter providing a maximum matching score from the obtained matching scores; a sectional shape generating unit which generates an optimum pattern sectional shape corresponding to the optimum process parameter; and a measurement unit which measures the optimum pattern sectional shape.