The present study is concerning the concept of units. This has the purposes to investigate whether the lack of the concept of units is seen not only in a area of length, but also in areas of weight and quantity of water, and whether the decalage in acquisition of the concept of units is seen between the three areas. Subjects are eighty elementary school children from first grade to fourth. They are given the three tests individually. The tests are concorning the concept of units in length, weight and quantity of water. For example, the test of length uses two tapes of paper. They are cut off in n_1 and n_2 number's segments respectively. That is, the situation of the test is made up that children fail to judge the relative length of two tapes, when they judge it by number of segments. In this way, the test of weight uses two clay balls and that of quantity of water uses two beakers with water. The main results are as follows. 1.The lack of concept of units is seen not only in length, but also in weight and quantity of water. And the amount of the lack of the concept is different between the three areas. Length has less lack of it than weight, and weight has less lack than quantity of water. 2.The decalage in the acquisition of concept of units is seen between areas of length, weight and quantity of water. Length is first in the asquisition of concept of units, weight is second and quantity of water is third.