Piaget, J. et al.(1960) suggested that the number of segments (elements) of two lines might influence the judgement of their lengths at a certain level, although its role was equivocal. And then they said that the youngest subjects lacked the concept of number so that their judgements of length were not influenced by the number of segments. The present study was aimed at investigating. their suggestions systematically, including development of number, with a hundred children from three to eight years old. The children were given the six tasks individually. 1) Counting objects. 2) Taking out the given number of an aggregate. 3) Comparison of two number words. 4) Conservation of number. 5) Comparison of length-A (Conservation of length, arranging matches at right angles to one another and in a series of zigzags.) 6) Comparison of length-B (using two straight strips of paper, each having different number of segments.) The findings and suggestions which were gained in this experiment were as follows: 1) The older children (six-seven years old) responded to the conservation task of number and the comparison task of length-A (conservation task of length) more correctly than the younger. On the other hand, the children of these ages responded to the comparison task of length-B less correctly than the younger. These results mean, if the children assure conservation of number, their judgements of length are influenced by the number of segments. And they also mean that the positive influence appears when two lines with the same size of segments are compared and the negative influence appears when two lines with the different size of segments are compared. 2) when these findins are compared with the findings by Gal'perin, P. Ya. et al.(1960) concerning conservation of volume, we can find these number influences on judgement of not only length but continous quantity in general. 3) The author suggests that the negative influence phenomenon arises from the concepts of number which children develop on the base of discontinous quantity and therefore in order to extinguish this phenomenon, children should develop the concepts of number on the base of continous quantity.