PURPOSE: The titled improving and treating agent, containing diltiazem or an acid addition salt thereof as an active constituent, usable continuously for a long term with almost no side effect, and administrable until there is no fear for recurrence.

CONSTITUTION: An improving and treating agent containing diltiazem: d-3-acetoxy-cis-2,3-dihydro-5-[2-(dimethylamino)ethyl]-2-(p-methoxyphenyl)-1,5-benzothiazepin-4(5H)-one which is a useful medicine having enhancing action on coronary circulation or an acid addition salt thereof as an active constituent. The remedy is capable of curing Raynaud's diseases, and removing the Raynaud's symptoms for Raynaud's syndromes or collagen diseases, etc. The dose thereof is 1W8mg/ kg/day, preferably 2W7mg/kg/day.