A method of transmitting a signal using code division multiplexing (CDM) in order to reduce the influence of channel estimation error in a multi-input multi-output antenna (MIMO) system using transmit diversity is disclosed. That is, the method of transmitting a signal by a transmitter using a plurality of antennas includes code division multiplexing input symbols using codes according to a predetermined spreading matrix, and transmitting the code division multiplexed symbols via the plurality of antennas according to an Alamouti scheme. At this time, the predetermined spreading matrix is set such that a first component and a second component configuring interference due to channel estimation error have orthogonal phases.