The age related slowing in cognitive processing speed was investigated with a mental rotation task. Slope of regression line predicting reaction time from rotation angle was assumed to be an index of mental rotation speed, and the intercept increase when the task was changed standard to mirror image, an index of decision process speed. Three groups were compared: young control (mean age = 19.1), younger elderly (M = 69.5) and older elderly (M = 79.0). In the mental rotation speed, an age difference between the control and older groups, but not between the older groups, was found. However, the decision process speed differed no only between the control and older groups, but also between the two older groups. These findings indicated that the effect of aging was larger on decision process than on mental rotation.