While TCP is highly successful in the wire-line Internet, its performance fast degrades as the number of hops increases in multihop wireless networks. It is due to not only the half-duplex nature of the wireless medium, but also the congestion spreading phenomenon. Congestion in one wire-less link spreads over space rather than localized to a link, causing interference to packet transmissions on neighboring links. Therefore, the space-shared feature of multihop wire-less network makes congestion control different from that in wired networks. Since TCP often errs in estimating congestion level due to the wireless interference, it can overly inflate the transmission window and blast packets into the network, resulting in high level of congestion. We propose a novel algorithm to detect congestion in multihop wireless networks, which enables TCP to adjust the window size precisely. Performance evaluation through simulations confirms the advantage of our proposal in detecting spatial congestion in multihop wireless networks.