The object is to provide a stable SM-108 derivative, which is effective as a carcinostatic agent, particularly an SM-108 derivative having good storage stability. An SM-108 compound having good storage stability can be produced by producing an organic sulfonic acid salt compound of SM-108. Further, a crystalline SM-108 compound containing a trace amount of an organic carboxylic acid can be produced by using an aqueous solution of the organic sulfonic acid salt of SM-108, and by adding an alkali metal salt of an organic carboxylic acid to the aqueous solution to neutralize the aqueous solution and then causing the crystal precipitation in the solution.