This study investigated how our recognition space changes due to acquisition of categorical knowledge. Thus far, many studies on categorization have used arbitrary features as relevant dimensions for categorization. In our study we defined the relevant dimension as the principal component feature that is found in human recognition space. Recognition space was examined by two tasks, a similarity estimation task and a task in which a momentary stimulus is presented for identification. When the task was similarity estimation, after category learning, the recognition space changed as the stimuli which belong to the same category concentrate on one point. But in the identification task, which is thought not to be affected by higher level knowledge, this type of strong change was not observed. And the recognition space change was not a simple change in the relevant dimension, but rather each category group changed independently.