We investigate theoretically the electron spin relaxation and decoherence rates in a single quantum disk and clarify their dependence on the magnetic field strength, the temperature and the strength of quantum confinement. In the weak confinement regime, the spin relaxation rate at low temperatures is dominated by the two-phonon processes and its dependence on the magnetic field is rather weak. In the strong confinement regime, on the other hand, the spin relaxation rate at low temperatures is governed by the one-phonon processes and exhibits the characteristic B4 dependence on the magnetic field (B). Non-Markoffian theory is developed for the electron spin decoherence rate due to the electron-phonon interactions based on the double Feynman diagrams. The dependencies of the electron spin decoherence rate on the magnetic field and the temperature are clarified.