It is generally considered that a fault-tolerant system is composed of a hardware system and a software system. The present work assumes that a hardware system is composed of two units (processors). Two software maintenance policies depending on the priority maintenance for software are proposed. In particular, software maintenance policies are discussed by evaluating the (time dependent) transient solutions of the reliability measures. Formulating Markov process models for two policies, and calculating the reliability measures numerically, two software maintenance policies for the resulting reliability measures are compared.