In this paper, we propose an adaptive handoff algorithm based on the "threshold-with-hysteresis." In general, such handoff algorithms are proposed to improve the handoff decision for a specific user. Our algorithm aims to provide effective network utilization for all users. To realize the handoff decision based on the mobile nodes, the agent advertisements include information of the congestion status at the base station in the algorithm. For implementing the algorithm to Mobile IP, we also propose the system architecture with a QoS mechanism. The simulation experiment shows that our proposed algorithm improves the average handoff blocking rate than the previous method in spite of low over-head of our expansion.