A method for transmitting a second demodulation reference signal added to support an additional antenna port (referred to as a “second antenna” hereinafter) in a downlink subframe having an extended cyclic prefix (CP), in which a resource (referred to as a “first resource pattern” hereinafter) to which a first demodulation reference signal will be mapped is configured, through a predetermined number of antenna ports (referred to as a “first antenna” hereinafter) in a wireless communication system includes: specifying positions of resources to which a channel state information reference signal (CSI-RS) is mapped in subframe(s) in which the CSI-RS is not transmitted and a subframe in which the CSI-RS is transmitted on the basis of CSI-RS configuration information; selecting a resource (referred to as a “second resource pattern” hereinafter) to which the second demodulation reference signal will be mapped from resources of the subframe(s) in which the CSI-RS is not transmitted, the resources corresponding to the positions of the resources to which the CSI-RS is mapped; and mapping the second demodulation reference signal to the second resource pattern.