A high speed Viterbi detector for an optical disk system, includes a frequency dividing unit for generating an auxiliary clock at one-third of the main clock frequency, a branch metric calculation unit for calculating each of a plurality of branch metrics, a serial-to-parallel converting unit for outputting each of the branch metrics at the main clock frequency in units of 3 state bits, an addition/comparison/selection unit for adding the branch metrics and previously stored state metrics and for comparing the addition results to select and output the minimum of the addition results as a new state metric, and for outputting a corresponding path selection signal, at the auxiliary clock frequency, a path memory for storing the path selection signal and for outputting parallel data corresponding to the path selection signal, at the auxiliary clock frequency, and a parallel-to-serial converting unit for converting the path memory output into serial data.