Aerospike nozzles have been expected as thrust nozzles for SSTO vehicles, because they show high performance in wide range of altitude or pressure ratio. Sidewalls for linear aerospike nozzles are indispensable to prevent the undesirable spanwise expansion of jets, which reduces the thrust. However, oversized sidewalls increase the weight of nozzles, and it is important to optimize the shape and the size of sidewalls. In this study, the flow field structures around the linear aerospike nozzles are visualized by using the laser induced fluorescence (LIF) technique and the pressure distributions on the nozzle spike are analized by the pressure sensitive paint (PSP) technique. The optimized shape of sidewalls is suggested by comparing the pressure distributions on the nozzle spike with three types of sidewalls and that without sidewalls.