1. A magnetic data processing device comprising: an input part that sequentially receives magnetic data output from a magnetic sensor; a storage part that stores a plurality of the magnetic data as a data set of statistical population; an index derivation part that derives a distribution index of the data set of the statistical population; a reliability determination part that determines whether or not reliability of the data set of the statistical population is acceptable based on the distribution index and a decision criterion; and a decision criterion setting part that increases 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 decreases 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.