Daisu-gaku Seigi (Commentary on Algebra; a preparatory book for entrance examination in algebra) written by Seiji Iwakiri was not only useful for passing entrance exam but also excellent as an inspiring mathematics study book. Iwakiri established a self-learning drill style of an exam prep book and strictly introduced a way of careful checking of dividing by zero in problem solving in Daisu-gaku Seigi. Iwakiri transplanted this drill style and way of checking zero from those in higher education to the field of exam prep books. In this paper we discuss mathematics-educational significance of Daisu-gaku Seigi by comparing with Fujimori's exam prep book which was very popular in that age.