This paper aims to critically examine the diffusion model for the four-pillars type (Sajusik) house, which has been interpreted as a result of Baekje expansion. I focus on the regional-radical diffusion process of the Sajusik house, suggesting the expansion of the regional trade/exchange network and the diffusion of material culture in Mahan society as an alternative model. Beginning by raising the question of the temporal location of the Jangsan-ri site in Cheonan, I investigate the internal area ratio of the four-pillars and radiocarbon dates. As a result, it was identified that the Sajusik house had a regional-radical diffusion process rather than gradual-sequential proliferation. It seems that the architectural style of the Sajusik house had spread rapidly throughout Mahan as the exchange between the two regions in the third century CE suddenly increased after having undergone a separate and independent development process in Hoseo and Honam areas. The direct cause of this is the expansion of trade/exchange networks among Mahan polities. In order to alleviate the disparity in access to resources between regions, it seems that the Mahan elites exercised a network-based interaction strategy, and based on this, a cooperative and peaceful interaction network would have been established in Mahan society.