To provide a method for heaping up a heat insulating layer in a roof-floor heat-insulated structure, which can inexpensively, easily and additionally increase the thickness of the heat insulating layer on a top surface of a roof-floor building frame of a concrete-based building having already undergone the construction of the roof-floor externally-heat-insulated structure.

Characteristically, this method for heaping up the heat insulating layer in the roof-floor heat-insulated structure having a structure, in which a composite panel is held in a sandwiched state using an adjuster, includes: a step of detaching the composite panel 5, held in the sandwiched state using the adjuster 4, from the adjuster 4; a step of placing a new prepared plate-like heat insulating material 30 for heaping up the heat insulating layer, on the adjuster 4 from which the composite panel 5 is detached; a step of superposing and sandwiching the existing composite panel 5, detached from the adjuster 4, on the top surface side of the plate-like heat insulating material 30 for heaping up the heat insulating layer, placed on the adjuster 4; and a step of detachably attaching an adjuster spacer 31 for securing a height equivalent to a heap-up height, onto the top of the adjuster 4.