This paper presents a mechanism that aggregates the bandwidths of multiple network interfaces on computers in mobile environments. Research and development on wireless technologies has recently been extremely active. As a result, various kinds of wireless media have been emerging and these have been installed on computers. However, the demand for broader bandwidths has not yet been met. We designed and developed a socket-level bandwidth aggregation mechanism (SBAM) that offer broader bandwidths using multiple wireless interfaces on computers. The most distinctive feature of SBAM is that the mechanism is implemented as a feature of operating systems. This involves two benefits. First, it is possible to deploy SBAM gradually in computers on the Internet. If the bandwidth aggregation mechanism (BAM) is achieved in a network stack, it must be implemented in all computers on the Internet. However, it is not necessary to implement it in all computers on the Internet, since it can recognize its own existence on peer hosts and determine whether to use BAM or not. Second, it does not require modifications to most existing software. For example, if the mechanism is achieved as a feature of a transport protocol, such as TCP, all applications using TCP must be re-written to adapt to the new transport layer protocol. However, SBAM does not require this, since the socket hides the existence of the mechanism. We evaluated SBAM in an actual wireless network environment. We achieved an increase in throughput by a factor of 1.6. SBAM offers broader bandwidths for applications by utilizing various wireless devices on computers while it avoids modifications to existing environments.