The relaxation of ultrathin polymethyl methacrylate (PMMA) films, which were spin-coated on SiO2 and SiOH substrates, was examined with different film thicknesses. In the glassy state, several relaxation processes revealed temporal variations in thicknesses. In sharp contrast to bulk PMMA in which the monotonic volume reduction was expected, the ultrathin films exhibited thickness variations, which would be classified into at least four types: (1) a slow increase in the thickness with time, where the thickness, h, is less than the radius of gyration, R-g (h < R-g); (2) scattered, fluctuating temporal variation (h similar to R-g); (3) contraction in film thickness, followed by expansion for the films with h similar to 2.6R(g); and (4) scattered temporal variation eventually leading to bulk relaxation (h > 3R(g)). Such a complex relaxation phenomenon could have originated from the interaction of the substrate with the heterogeneous out-of-equilibrium configurations of the polymer molecules that were confined to a quasi-2-D geometry.