An intracellular calcium concentration increase inhibitor containing as the active ingredient (1) a boron compound represented by the formula (I). ##STR00001## The compound represented by the formula (I) inhibits the increase of the intracellular calcium concentration, and therefore it is deemed to be useful as an agent for the prophylaxis and/or treatment of platelet aggregation, ischemic diseases in hearts and brains, immune deficiency diseases, allorgosis, bronchial asthma, hypertension, cerebrovascular spasm, various renal diseases, pancreatitis, Alzheimer's disease, etc.