Various manuals are used for plant operations and maintenance for plants. Adequate employment of manuals enables to decrease human errors in the workplace, and to ensure the quality of operator's/worker's performance not to heavily rely on the expertise of them. Inadequate information on manuals, however, sometimes provokes human errors. For example, it is difficult for skilled operators/workers to grasp the points of manuals if the manuals describe as over-detailed. They may be hard to recognize the information slightly different from the one that he/she acquires in his/her long experiences of the tasks. On the other hand, a novice operator or worker will make human errors if he/she uses a manual with rough descriptions of tasks because it may be difficult to understand the tasks due to his/her little knowledge and experiences for the tasks. Therefore, the information type of operation manuals should change depending on the knowledge and experiences of operators and workers for the tasks. This study experimentally investigates the relations among performance of tasks, information type of operation manuals, and skill level of operators. The results suggest that an operation manual describing concrete recovery actions for a task step by step will suit for a novice operator, while a skilled operator can use effectively the information of operation goal.