1. A sphere decoder for simultaneously detecting a plurality of symbols from a signal received through a communication channel, the sphere decoder comprising: an initial estimation unit for obtaining an initial estimate of a transmitted symbol vector from the received signal; an initial radius setting unit for calculating a Euclidean distance between a lattice vector corresponding to the initial estimate of the symbol vector and the received signal, and for setting the Euclidean distance as an initial radius; a lattice search unit for searching a set of candidate lattice vectors included inside a hypersphere with the initial radius, centered at a received signal vector; and a maximum likelihood estimate determination unit for selecting a lattice vector having a minimum Euclidean distance from the set of candidate lattice vectors.