This article is a case report of the cognitive counseling of an elementary school student who was weak in math. He was a fifth-grader, and had some difficulty in the division of decimals and solid geometry. We analyzed his errors in solving problems in those fields, and tried to correct his inadequate knowledge that caused such wrong answers. Our assistance resulted in the improvement of his performance in solid geometry, but this effect was not clear in decimals, probably because our intervention did not change more fundamental knowledge. Further intervention should be based on the detailed assessment of his performance and analysis that clarifies what knowledge is responsible for his difficulties in decimals.