PURPOSE: To obtain the title filler which can easily perform sealing and can serve also as a deformable object, by heating a mixture of a finely divided synthetic raw rubber with a lubricant to a specified temperature.

CONSTITUTION: A synthetic raw rubber such as ethylene/propylene rubber, silicone rubber or nitrile rubber is finely divided to obtain a finely divided rubber (a). Component (a) is mixed with a lubricant (b) such as grease or oil in such an amount as to cover the periphery of component (a) to obtain a mixture. This mixture is heated to a temperature as high as that at which component (a) begins to melt and is kneaded into a semi-fluid to obtain the title filler M. The filler M is poured into the case 10 of the body 1 of a temperature- sensitive actuator A to form a heat-expandable body 2 which performs sealing and optionally serves also as a deformable body 4. When it serves also as a deformable body 4, a diaphragm 5 can be omitted.