  Semi-simple unification is one of a model which naturally solves two
difficulties in the supersymmetric grand unification theory: doublet-triplet
splitting problem and suppression of dimension 5 proton decay. We analyzed the
dimension 6 proton decay of this model using perturbative analysis at the
next-to-leading order. The life time of proton is 3 \times 10^{34} - 10^{35}
years for wide range of SUSY breaking parameters, and there is an intriguing
possibility of observing proton decay signals in the next-generation water
Cherenkov detectors such as Hyper-Kamiokande and TITAND. Several uncertainties
in this prediction are also discussed.
