A catalyst for preparing polyester resins from monobasic or polybasic acids and polyhydric alcohols, comprising a Magnesium compound which has a bonding portion represented by formula —Mg—O—(CR12)a-Q, wherein Q is a Nitrogen or Sulfur and a is an integer from 1 to 4, is disclosed. Catalyst may further comprise Titanium compounds, for example titanium alkoxide, wherein titanium diisopropoxy bistriethanolaminate is particularly preferable from the viewpoint of reactivity and coloring property. Catalyst is capable of shortening production time for polyester resins.