In this paper, we considered two networks which are the next-generation public network and the Internet, as the future information network infrastructure. These two networks had originally been developed independently, and now they both have several services for users to be able to utilize services on the Internet environment from the public network environment, and especially to utilize services on the public network from the Internet environment. This will have a bad influence on the future multimedia network society. Consequently, we adopted TINA, and proposed the interworking architecture for seamless service utilization between the two networks. Because these two networks have different characters, we introduce an IWU as a gateway node to realize seamless interworking. The IWU is the gateway for both application level and transport level. More over, considering to utilize public network's services from the Internet after a user moves to the place where s/he can only use the Internet environment, the kind of terminals/operating-system s/he would use is indefinite factor. However, it is obviously desirable to be able to utilize the public network's services with any terminal/operating-system. Therefore we also proposed and implemented the platform named PLUS-TINA. The PLUS-TINA is constructed with Java language so as to insure its portability.