We investigate Seifert surgery along a knot in a homology 3-sphere
by using Reidemeister torsion. We suggest that it may be better to classify
Seifert surgery into three types such as norm one type, rational type and
norm zero type by corresponding the result of surgery to a branched cyclic
covering space. In rational type case, a value of the Alexander polynomial
at a root of unity has an information of the indices of singular fibers.