The invention relates to an elastic polyurethane foam having a resonance frequency of up to 3.5 Hz and a process for producing same. This foam is obtained by using as main reactant materials at least one polyol selected from among specific polyoxyalkylene polyols and polymer dispersion polyols based on said polyoxyalkylene polyols as the matrix and a polyisocyanate compound. This foam is produced by using a polyoxyalkylene polyol having a particularly low hydroxyl value (X mg KOH/g) and a particularly low total unsaturation (Y meq/g) or a polymer dispersion polyol based on said polyoxyalkylene polyol as the matrix. More specifically the polyoxyalkylene polyol has a hydroxyl value (X mg KOH/g) of 5 to 38 and a total unsaturation (Y meq/g) of up to 0.07, with the proviso that Y « 0.9/(X-10). A preferred polyoxyalkylene polyol is one containing 70 wt.% or more oxypropylene groups and 5 wt.% or more oxyethylene groups. The use of a small amount of a crosslinking agent together with the polyoxyalkylene polyol is preferred. This foam has vibration properties suited for seat cushions of automobiles. Therefore, it can give a comfortable ride.