1. A method for measuring a measurement target pattern with an electron microscope to acquire an actual electron microscope image of the measurement target pattern under image acquisition conditions and estimate shape of the measurement target pattern by using the acquired actual electron microscope image, the method comprising: a measurement recipe creation step of obtaining pattern shape information by modeling approximate shapes of various target patterns in numerical data in advance, which are obtained by varying dimensions in a predetermined predicted variation range; a library creation step of creating first simulated waveforms of electron microscope signals concerning various target pattern shapes by performing electron beam simulations for the various target pattern shapes within the predicted variation range, the electron beam simulations representing characteristics of an electron microscope, including electron optics resolution, beam divergence half angle and aberration coefficients, and image acquisition condition imposed by the electron microscope, and storing a combination of the created first simulated waveforms of the electron microscope signals and the pattern shape information, which correspond to the first simulated waveforms and are obtained in the measurement recipe creation step, as a library; and a measurement step of acquiring an actual electron microscope image of a measurement target pattern with the electron microscope which is adjusted in focus and astigmatism and selecting a first simulated waveform or a second simulated waveform which is most similar to the acquired actual electron microscope image by comparing the acquired actual electron microscope image with the first simulated waveforms of the electron microscope signals created in the library creation step, or second simulated waveforms which are created from the first simulated waveforms, and estimating the shape of the measurement target pattern from the pattern shape information that corresponds to the selected first simulated waveform or second simulated waveform and is used in the library creation step, wherein the library creation step includes the steps of: calculating a best focus position by using simulation images for each of the target pattern shapes in the same evaluation method as with an actual electron microscope, in advance; and reflecting the calculated best focus position in the electron beam simulation as an image acquisition condition imposed by the electron microscope.