An abnormality determination method includes: a first distance calculation step of calculating, based on measurement data of an object, a first Mahalanobis distance in a first unit space corresponding to a normal mode of the object; a second distance calculation step of calculating, based on the measurement data, an (i+1)-th Mahalanobis distance in an (i+1)-th unit space corresponding to an i-th abnormal mode of the object for each integer i greater than or equal to 1 and less than or equal to N, N being an integer greater than or equal to 2; and an abnormality determination step of determining, based on the second to (N+1)-th Mahalanobis distances, one of the first to N-th abnormal modes where the object is when the first Mahalanobis distance exceeds a threshold value.