To provide a method for producing a heat insulating sheet for a fireproof garment excellent, firstly, in air permeability and capable of mitigating stuffiness in the garment, having, secondly, high heat insulating properties, and excellent, thirdly, in workability (easy movability), such as lightweight properties and pliability.

This method for producing the heat insulating sheet 4 comprises together mixing polyimide balloons 5, a binder 6, and additives 7, to give a mixture, forming the mixture into a sheet, coupling the polyimide balloons 5 with each other at contacted areas 5', by curing the binder 6, to form polyimide foamed materials 8, then removing the additives 7 from the sheet-shaped polyimide foamed materials 8, to prepare and form air-permeating voids 9 in spaces from which the additives 7 are removed, so that the heat insulating sheet 4 is produced, wherein the method firstly adapts the polyimide balloons 5 as a material for the heat insulating sheet 4. Further, the heat insulating sheet 4 is subjected to quilting processing, and then used for a lining as an inner layer of the fireproof garment.