1. A T-wave alternans (TWA) measuring electrocardiograph comprising: a transformation coefficient storing section configured to store a transformation coefficient for deriving a Frank's vector electrocardiogram; an electrocardiograph controlling section configured to produce a scalar electrocardiogram from electrocardiographic signals of measurement electrodes adapted to be attached to a subject, and configured to derive the Frank's vector electrocardiogram from the scalar electrocardiogram by using the transformation coefficient; and a TWA measuring section configured to measure a presence of TWA based on the derived Frank's vector electrocardiogram, wherein the transformation coefficient storing section comprises: a personal coefficient database configured to store a personal coefficient that is acquired from the subject and that is specific to the subject, as the transformation coefficient; and a group coefficient database configured to store, as the transformation coefficient, a group coefficient that is an average of a plurality of transformation coefficients acquired from an unspecified number of persons of a statistically effective population.