We study phase separation of quenched polymer solutions under an external shear flow by using a stochastic molecular dynamics method. We observe the formation of a stringlike pattern of polymer domains normal to the direction of the shear flow. The characteristic size of the layered domains has a power law relation with the shear rate. Shear-induced rheological behavior of the mixture is also examined in terms of the excess viscosity. We find that the maximum excess viscosity scales as the shear rate, and that the numerical value of the power-law exponent agrees well with theoretical scaling predictions.