This study examined the errors of compare word-problems. 81 fifth graders solved the word problems individually according to a computer program. After subjects planned their solution, the computer asked the student to tell the reason of their plan. As a second task, the computer examined each solution, and asked the subjects some questions to investigate how they understood the problem in the case of an error. We investigated how they planned their solution from their verbal reports, and we clarified the part of the problem solving process where the errors were made considering their responses to the questions. We found that many wrong solutions were due to the use of key words such as ‘times’, and that many subjects misunderstood the structure of the problem. We also discussed the problems to design a computer-based tutoring system for word problems.