Mobile devices such as smartphones have various restrictions in terms of resources such as batteries and processors. The load reduction on the mobile devices is possible by offloading, migrating application to the resource-rich environment such as cloud servers. However, further study is required on multi-tier environment which contains multiple resources composed of edge servers and cloud servers. This paper proposes an offloading system based on estimated response time (ERT) in multi-tier environment which contains multiple resources. The proposed offloading system calculates ERT based on the state of network and computation resources of each execution resource. Offloading is executed on execution resources with the minimum ERT. We implemented the prototype of the proposed offloading system and evaluated the performance.