1. A shape measurement method comprising the steps of: applying one of an electromagnetic wave and a beam of charged particles to a surface of a specimen, using an irradiation unit that moves along an axis parallel to a scanning direction relative to the surface of the specimen; measuring a signal intensity of one of an electromagnetic wave reflected from the surface of the specimen and a beam of charged particles generated from the surface of the specimen as a result of irradiation from the irradiation unit; calculating a slope angle of the surface of the specimen at a position irradiated with one of the electromagnetic wave and the beam of charged particles on the basis of the measured signal intensity; determining candidates for cross-sectional shape of the specimen on the basis of the calculated slope angle; estimating a signal intensity of one of an electromagnetic wave that would be reflected from a surface having a cross-sectional shape of each of the candidates and a beam of charged particles that would be generated from the surface having a cross-sectional shape of each of the candidates if an angle of incidence of one of the electromagnetic wave and the beam of charged particles with respect to the surface having a cross-sectional shape of each of the candidates were changed to a specific angle of incidence different from an angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen; comparing the estimated signal intensity with a signal intensity obtained by measurement performed when the angle of incidence of one of the electromagnetic wave and the beam of charged particles applied to the surface of the specimen is changed to the specific angle of incidence; and determining the cross-sectional shape of the specimen on the basis of a result of the comparing step.