Automated construction is in high demand in many applications in the extreme environments. Fully automatic construction system is desired but difficult to realize for many reasons. We have proposed a novel concept of full-automated construction called Automatic Modular Assembly System (AMAS), which drastically reduces the difficulty of automated construction task. In this paper, we focus on distributed control method of the assembler robots. We assume large number of modules and robots; therefore we need to organize the motion paths of these robots to avoid collision among them. Moreover, the area on which the assembler robots can move dynamically expands during the construction process. In order to optimize the complex coordinated motion among the assembler robots, we introduce a distributed algorithm based on a gradient field. The structure modules generate the gradient field by using neighbor-to-neighbor communication, and the robots are led by the field. Incorporating some additional rules for the field and robots, we can make a kind of self-organized flow on the system, and it enables collision-free and time-effective assembly. Some results of simulations give quantitative evaluation of the algorithm.