As for the development of proportionality concept, Inhelder & Piaget (1955) mainly administered adjustment tasks and proposed domain generality. On the contrary, Siegler (1981) and other researchers used comparison tasks and proposed domain specificity. This study used both tasks and examined domain specificity among children's solutions to two proportionality tasks: velocity and thickness. Adjustment tasks and comparison tasks about velocity and thickness were administerd to 144 pupils from third to sixth grades. Task scores, stages identified by reaction patterns and justifications, and solution strategies were analyzed. The results showed that reasoning in adjustment tasks (proportional reasoning) was domain general at each grade, and that reasoning in comparison tasks (comparison of intensive quantity) was domain specific from fourth to sixth graders. Domain specific sub-process (comparison process) and relatively domain general sub-process (relation-representation process and calculation process) were discussed.