This paper deals with a method of keeping production smoothing in JIT production systems with machine breakdowns. In general, when the production smoothing is disturbed by machine breakdowns in a JIT production system, all processes are suspended in order to keep the production smoothing. However, if the suspended time becomes very long, the production cost becomes large. To cope with the problems occurred in the production smoothing system, we propose a method to minimize the production cost and to keep the production smoothing. In this paper, we develop the two kind models of production smoothing systems and evaluate the method by computer simulations.