Dynamical properties of the magnetic B12 cluster compound HoB22C2N were investigated. HoB22C2N is taken to be representative of the class of trigonal and rhombohedral B12 cluster compounds which were found to exhibit spin-glass behavior as measured by dc superconducting quantum interference device magnetometer and specific-heat measurements. These are examples of magnetic glassiness being observed in rare earth boron-rich crystalline cluster compounds. Well defined maxima in the in-phase linear ac susceptibility χ' curves were observed, indicative of the spin-glass transition. Strong frequency dependence of the cusp temperature Tf was found. The dependence of Tf could not be analyzed satisfactorily by the dynamical scaling theory of a three-dimensional spin glass. A more detailed investigation of the behavior of relaxation times by Cole-Cole analysis showed that the behavior in HoB22C2N is different from the simple blocking of a superparamagnetic system, because of the temperature dependence of the parameter α representing the width of the relaxation-time distribution function g(τ). The median relaxation time was also determined and the data were found to be described well in terms of a generalized Arrhenius law ln τ∝T-2.5. These results indicate that HoB22C2N is a two-dimensional spin-glass system, which supports what has been speculated previously.