narrow enough to be invaded by the granules e g a space up to about 0 1 mm for talc etc it has a function to mitigate the mechanical impacts on the hollow ceramic body 42 due to its elasticity as well as to provide sealing and thermal insulating functions if the o ring packing 34 used in the embodiment 3 is preliminarily retained on the end side of the collar portion 42c close to the cylinder and the aforementioned space is sealed the inorganic thermally insulating granules 44 can be packed even when the space is wide the arrangement 41 for thermally insulating the precombustion chamber is press fitted into a hollow space of a cylinder head not shown the glow plug 20a and the fuel injection nozzle 20b are inserted into respective bores 42a and 42b in the arrangement 41 after an engine incorporating the present apparatus was operated at 4000 rpm for 100 hours the precombustion chamber was observed by lifting the cylinder head it showed no defects at the ceramic members no tilting descent indentation or deformation of the thermally insulating arrangement upon disassembling the arrangement 41 for thermally insulating the precombustion chamber it has been found that there was no sign of invasion of combustion gas into the space between the inner surface of the fitting member 47 and the collar portion 42c so that complete sealing was maintained referring now to fig5 there is shown a further embodiment.