In batch processes, the processing times of operations may fluctuate due to unforeseen failure of equipment or uncertain quality of the raw materials. Such disturbances affect the starting times of successive batches and force operators to do additional tasks and/or to change the processing order of pre-assigned tasks. Unexpected changes of schedule induce human errors, and affect the schedules for preparation of raw materials, inspection of products and product delivery. Thus, the propagation of such disturbances to successive batches should be suppressed as soon as possible. Usually, the buffer time, which is idle time between batches, is used to adjust the delay caused by the disturbances. However, it is not clear how to assign buffer times between batches. In this research, a systematic procedure deciding buffer times is proposed. First, a method of calculating the influence of the delay of an operation on subsequent batches is explained. Then, a method of calculating buffer times that can absorb every pre-assigned disturbance within a given number of batches is proposed. In a real process, regular processing times of some operations are decided so as to have some allowance, and the processing times can be shortened by using additional manpower or utilities. The methods of shortening processing times are classified into several strategies, and a procedure for assigning buffer times under the predetermined strategies is also proposed. The effectiveness of the proposed procedure has been verified by simulation study.