1. A magnetic data processing device comprising: an input part configured to sequentially receive magnetic data output from a magnetic sensor; a storage part configured to store a plurality of the magnetic data as a data set of a statistical population; an index derivation part configured to derive a distribution index of the data set of the statistical population; a reliability determination part configured to determine whether or not reliability of the data set of the statistical population is acceptable based on the distribution index and a decision criterion; an offset derivation part configured to derive an offset of the magnetic data based on the data set of the statistical population when the reliability determination part determines that the reliability of the data set of the statistical population is acceptable, and configured not to derive an offset of the magnetic data when the reliability of the data set of the statistical population is unacceptable; and a decision criterion setting part configured to increase strictness of the decision criterion when the reliability determination part determines that the reliability of the data set of the statistical population is acceptable and configured to decrease the strictness of the decision criterion when the reliability determination part determines that the reliability of the data set of the statistical population is unacceptable, wherein the decision criterion setting part is configured to change the decision criterion in a stepwise manner each time the reliability determination part determines whether or not the reliability of the data set of the statistical population is acceptable, wherein the decision criterion setting part is configured to fix the decision criterion when the decision criterion converges within a predetermined range, and wherein the decision criterion setting part is configured to determine whether or not the decision criterion converges within the predetermined range according to a ratio between a number of times of determination in which the reliability is determined to be acceptable and a number of times of determination in which the reliability is determined to be unacceptable while the reliability is determined by the reliability determination part a predetermined number of times.