In order to examine the teaching method of arithmetic word problems, we created investigation problem for first graders based on Riley's classification and analyzed the relation between the drawings and the formula for solution which the subjects drew in the problem solving process of word problems. As a result, it was found that, in order to answer arithmetic word problems correctly, comprehension of relation was required in problems of CHANGE 1 (a+b=x) and 2 (a-b=x), and comprehension of unknown value was required in problems of CHANGE 3 (a+x=b), 4 (a-x=b), 5 (x+a=b) and 6 (x-a=b), where a and b denote known values and x denotes unknown value. These findings suggested that it is important in instruction of arithmetic word problems to make problem structure grasped after clarifying the relation and the unknown value.