The present invention relates to a method for producing a formyl group-containing porous base matrix, comprising the steps of introducing a spacer in a formyl group-containing porous particle; and then oxidizing the spacer with periodic acid and/or a periodate, to transform the part of the spacer into a formyl group; wherein the formyl group content in the porous particle after introduction of the spacer is not more than 3 μmol per 1 mL of the porous particle. Also, the present invention relates to a method for producing an adsorbent, comprising the step of immobilizing an amino group-containing ligand on the formyl group-containing porous base matrix. According to the present invention, a formyl-group containing porous base matrix and an adsorbent produced from the porous base matrix of which adsorption amount is high and which is has high strength and of which ligand is difficult to be leaked are provided.