The cloud computing is a computing paradigm that users can rent computing resources from service providers as much as they require. A spot instance in cloud computing helps a user to utilize resources with less expensive cost, even if it is unreliable. In this paper, we propose the workflow scheduling scheme that reduces the task waiting time when an instance occurs the out-of-bid situation. And, our scheme executes user's job within selected instances and expands the suggested user budget. The simulation results reveal that, compared to various instance types, our scheme achieves performance improvements in terms of an average execution time of 66.86% over shortest execution time in each task time interval. And, the cost in our scheme is higher than an instance with low performance and is lower than an instance with high performance. Therefore, our scheme is difficult to optimize cost for task execution.