1. An aberration evaluation method for evaluating aberration of an irradiation system based on an image obtained by scanning an electron beam onto an aberration evaluation pattern, the irradiation system emitting the electron beam, the aberration evaluation method comprising: a focus setting step of setting a focus position of the irradiation system onto the aberration evaluation pattern, wherein the aberration evaluation pattern includes a periodical structure having a prescribed line width and a prescribed distance between the lines, the periodical structure being provided on the same plane and formed outward from a center that is an irradiated position of the electron beam when the electron beam is not deflected, the periodical structure being formed to extend 360 degrees around the periphery of the center and has either a concentric-circle figure or a spiral figure whose center is positioned at the center that is the irradiated position of the electron beam when the electron beam is not deflected, wherein the focus position is set onto the aberration evaluation pattern based on an electron signal obtained by scanning an underfocused electron beam onto the aberration evaluation pattern and an electron signal obtained by scanning an overfocused electron beam onto the aberration evaluation pattern, wherein the focus setting step includes: a first substep of scanning an underfocused electron beam and an overfocused electron beam onto the aberration evaluation pattern in a third and a fourth directions, the third and fourth directions being different from each other; and a second substep of setting the focus position onto the aberration evaluation pattern based on the maximum value of an electron signal obtained by scanning in the third direction and the maximum value of an electron signal obtained by scanning in the fourth direction, wherein, the first substep includes: a current value difference calculation step of calculating a difference between a first current value and a second current value, the first current value corresponding to the focus position of the irradiation system when the electron signal obtained by scanning in the third direction becomes maximum, the second current value corresponding to the focus position of the irradiation system when the electron signal obtained by scanning in the fourth direction becomes maximum, and a determination step of determining whether the difference between the first current value and the second current value is equal to or less than a predetermined value, wherein, in the second substep, when determining that the difference between the first current value and the second current value is equal to or less than a predetermined value, the focus position onto the aberration evaluation pattern is set based on the maximum value of an electron signal obtained by scanning in a fifth direction and the maximum value of an electron signal obtained by scanning in the fourth direction, the fifth direction being different from the third direction; and an evaluation step of evaluating aberration of the irradiation system based on electron signals obtained by scanning the electron beam onto the aberration evaluation pattern onto which the focus position of the irradiation system is set in a first direction and a second direction orthogonal to the first direction.