An abnormality determining apparatus includes an information storage unit for storing normal index information serving as an index of a normal state of the detection subject, an information obtaining unit for obtaining a plurality of types of information, and an abnormality determining unit for determining the presence of an abnormality in the detection subject on the basis of the normal index information stored in the information storage unit, and the information obtained by the information obtaining unit. The information storage unit stores a plurality of normal index information having different values. The abnormality determining unit selects, at a predetermined timing, normal index information to be used to determine the presence of an abnormality in the detection subject from the plurality of normal index information.