Apparatus and method for decoding a Space-Time Block Coded (STBC) signal. The decoding apparatus includes a channel estimator for estimating a real equivalent channel based on a coded signal; a channel converter for decomposing the real equivalent channel to a unit matrix and a subchannel; a receive signal converter for converting the coded signal to a real equivalent receive signal and converting the real equivalent receive signal to a converted receive signal based on the unit matrix; and a detector for detecting an estimate of a transmit signal by performing a maximum likelihood decoding using the converted receive signal and the subchannel. Since the transmit signal candidates are independent of each other, the complexity of the maximum likelihood decoding can be decreased. With the lowered complexity of the receiver, the power consumption for the decoding can be reduced and the high-speed data can be transmitted more easily in the actual mobile communication environment.