A curable composition includes (A) 100 parts by weight of an oxypropylene polymer having groups containing silicon atoms bonded to a hydroxy group and/or a hydrolyzable group and capable of crosslinking by forming a siloxane bond, a maximum Mw/Mn of 1.6, and an average molecular weight of at least 6,000, (B) from 100 to 200 parts by weight of calcium carbonate containing at least 30% by weight calcium carbonate of an average particle size of not larger than 0.5 .mu.m and surface-treated with a fatty acid, (C) from 30 to 100 parts by weight of a plasticizer containing at least 5% by weight phthalic acid ester plasticizer, (D) from 0.5 to 10 parts by weight of a compound represented by the formula R.sup.1 Si(OCH.sub.3).sub.3 and/or Si(OCH.sub.2 CH.sub.3).sub.4 (wherein R.sup.1 represents a monovalent organic group containing no amino groups), (E) from 0.5 to 10 parts by weight of a compound represented by following formula (1), and (F) from 0.5 to 5 parts by weight of an organotin curing catalyst, the total amount of components (D) and (E) being at least 2 parts by weight, and component (A) occupying from 15 to 35% by weight; R.sup.2 Si(CH.sub.3).sub.n (OR.sup.3).sub.3-n ( 1) wherein R.sup.2 represents a monovalent organic group having at least one amino group; R.sup.3 represents CH.sub.3 or CH.sub.2 CH.sub.3 ; and n represents 0 or 1.