The dielectric spectra of methyl m-toluate (MMT) in supercooled liquid and glassy states were measured over wide ranges of temperature T at ambient and elevated pressures P. We found that the frequency dispersion of the loss peak contributed by the structural alpha-relaxation is invariant to changes of P and T, while keeping the loss peak frequency f(alpha)(T,P) constant. This isochronal superposition property of the alpha-relaxation holds for different choices of f(alpha)(T,P). The invariant frequency dispersions for the same f(alpha)(T,P) are also indicated by the fractional exponent beta(KWW) in the Fourier transform of the Kohlrausch-Williams-Watts (KWW) function. Similarly, the fragility m index of MMT keeps approximately constant on varying pressure, largely different from H-bonded glass formers. The secondary beta-relaxation at a frequency higher than f(alpha)(T,P) is found to shift to lower frequencies by elevating pressure in concert with the alpha-relaxation. The ratio tau(alpha)(T,P)/tau(beta)(T,P) is approximately unchanged to variations of T and P while keeping tau(alpha)(T,P) constant. These properties observed in MMT offer experimental evidence of the dynamic correlation between alpha- and beta-relaxations in pure small-molecule glass-formers.