In this paper, we consider a new stochastic model for a file recovery action with checkpointing when the 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 earlier models by Gelenbe (1979) and Goes and Sumita (1995). We propose three kinds of approximation schemes to determine the optimal checkpoint interval which maximizes the 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 the present checkpoint model can reduce the system overhead which may occur in an unplanned system downtime.