For this study we are going to apply a type of computer tasks which are used in an actual control room. Subjects are supposed to perform the primary task (typing task watching the main monitor) and the secondary task (clicking targets with the mouse, which appear randomly in the sub-monitor) at the same time. The secondary tasks were differentiated with three different task intensities. During the experiment, the primary and secondary tasks were measured and evaluated depending on the intensities of the subtasks and the locations of the sub-monitors. Followings are the conclusions which were obtained after the evaluation: 1) The higher the task intensity is, the lower the task performance. 2) Regardless of the task intensity, the task performance of the subtask is high in the center of the monitor. When the task intensity is low, the center section of the monitor becomes the starting point for searching. 3) When the task intensity is high, the task performance in the left section of the monitor becomes low.