The sequences of disassembly/reassembly tasks for maintenance or repair have many variation in accordance with the task aim and the task site environment. The decision of the sequence has been left to an expert. However, due to the diversification of products, the shortage of experts is becoming a problem. It is important to accumulate the experts' knowledge of task sequences which include sequences for temporal tacking and mechanism adjusting tasks. In this paper, a knowledge representation method for the task sequences using a directed graph and a construction method of the task sequence knowledge using ATMS (Assumption base Truth Maintenance System) are proposed. A case study shows that the proposed task support system, which presents an appropriate task sequence and estimated task contents, is useful for unskilled workers to plan the disassembly/ressembly tasks.