To provide a silane-crosslinked (water-crosslinked) type flexible elastomer composition which is extremely soft and excellent in heat resistance.

This method for producing the above elastomer composition comprises each charging 100 pts.wt. hydrogenated styrene-butadiene-styrene polymer (SEPS) which is a base polymer, 300 pts.wt. plasticizer, 3 pts.wt. organic unsaturated silane, 1.5 pts.wt. organic peroxide and 1 pt.wt. antioxidant into a mixer 1 to carry out grafting, sheeting the grafted material in ribbon-like state, feeding the material while allowing to bite into a screw of a twin screw extruder 2, carrying out extrusion molding of the composition from a s spinneret 6 of an extruder 2 while adding a silanol condensation catalyst from a liquid pouring pump 5 and passing the mixture through a water tank and carrying out water crosslinking together with cooling.