PURPOSE: To obtain a new compound having excellent intraocular pressure- lowering action and useful as an excellent therapeutic agent for glaucoma.

CONSTITUTION: A compound of formula I (R1 and R2 are each H, OH, etc.; R3 and R4 are each H or R3 and R4 together exhibit a single bond; either one of R5 and R6 is H and the other is a protecting group removed in eyes) and its salt, e.g. griseolic acid-9'-pivaloyloxymethylester. The compound is obtained by reacting, e.g. a raw material compound of formula II with an one equivalent esterifying reagent of the formula R5' -X or R6' -X (X is a halogen, acetoxy, etc.) at a room temperature in a solvent such as DMF in the presence of 1,8- diazabicylo[5.4.0]undec-7-ene which is a deacidifying agent.