This paper proposes an determination algorithm of amplify-and-forward (AF) relay precoders and user receivers to minimize sum mean-squared-error (SMSE) for multi-relay aided multi-user (MRAMU) multiple-input multiple-output (MIMO) network. This algorithm is advantageous in that it can be executed with only local channel state information (CSI) at relays, meaning that a relay is aware of 2nd hop CSI of only the channel from itself to all users. Block diagonalization (BD) precoder at base station (BS) can make it unnecessary that 2nd hop CSI should be shared among relays. Given fixed BD precoder at BS, relay precoders and user receivers are designed as solution of optimization problem. We attempt to achieve suboptimal structures of relay precoders and user receivers based on sequential iteration and Karush-Kuhn-Tucker (KKT) theorem. Simulation results verify that the proposed algorithm provides the reduced SMSE compared to conventional simple AF (SAF) and minimum mean-squared-error (MMSE) relaying.