PURPOSE: To provide a treating method by which the forming surface of a mold for shell molding can smoothly be formed with high quality and good workability without contaminating the working environment.

CONSTITUTION: In this method for treating the forming surface of the mold for shell molding, a core 4 is arranged in a closed space 11 in which fine pulverized coating agent 3 is floated, and by executing sucking from the opposite side to the forming surface 41 of the core 4, the fine pulverized coating agent 3 is sucked from the forming surface 41 and filled in the gap among the grains consisting the core 4.