Surface-treated calcium carbonate characterized by giving, upon analysis by the method of mercury penetration, a void diameter distribution curve which has a peak for the most probable void diameter at smaller than 0.03 .mu.m and has a most probable void volume of 0.05 to 0.5 cm.sup.3/g; and a pasty resin composition containing the surface-treated calcium carbonate. The calcium carbonate preferably comprises one obtained by surface-treating calcium carbonate having a BET specific surface area of 10 to 100 m.sup.2/g with an unsaturated fatty acid (A) and a saturated fatty acid (B).