The purpose of this research is to realize a Multiple Robots System which can be easily developed and managed. In order to perform robust cooperative work, it is necessary that multiple robots can achieve not only a high level of cooperation without communication but also simple cooperation with communication, and then switch between modes according to the situation. We call such cooperative work ``Tight Cooperative Work''. To achieve such cooperative work, there must be an integrated concept in the system about structure in a robot and whole system. We consider Object Oriented System is effective to realize this concept. We propose a method for the construction of Object Oriented System for multiple robots. We constructed a cooperative working system called ICRoS and implemented it on two hexapod robots with parallel processors. The concept of Smalltalk-80 was applied as basic software of the system. We verified the effectiveness of this multiple robots system with four different experiments involving the cooperative lifting of a box.