Purposes of the present study were to reveal developmental processes of relational concepts among time, distance, and velocity. To achieve the purposes, the author not only prepared formally symmetric tasks as a whole for the three concepts, but also pursued symmetric expressions of attributes of the three concepts as much as possible. Children from 4 to 10 years old and adults were used as subjects. In this paper only data about the relation between time and velocity with constant distance were analyzed and reported. The main results were as follows: (a) In young children under 6 and a half years old, the relation between time and velocity seemed to be rather direct. In addition almost all children over 8 years old understood the inverse relation between time and velocity. (b) Gaining consciousness of and verbalizing processes of intuitive judgments were very difficult even when they could correctly judge the relation. (c) Most children under 9 years old seemed to judge based not on the time-distance-velocity system but based on the coordination of two by two relations.