In this paper, we consider a new stochastic model for file recovery action with checkpointing when a system failure occurs according to a homogeneous Poisson process. The present checkpoint model strongly depends on the system age and is quite different from the models by Gelenbe (1979) and Goes and Sumita (1995). We propose three kinds of approximation schemes to determine the optimal checkpoint interval which maximizes system availability, taking account of queueing effect due to idle periods in the transaction processing system. In numerical examples, the checkpoint model based on three approximation schemes is compared with earlier models quantitatively, and it is shown that it can reduce system overhead which may occur in unplanned system downtime.