To provide a water leakage prevention material and a water leakage prevention method, which prevent water leakage through repair of roof material which is laminated and fixed to a roof and has a fractured surface or a crack without replacement, and which enhance durability of roof material such as roof tiles and decorative slates.

A water leakage prevention material for a roof to prevent water leakage from a portion having a crack in a roof material laminated and fixed to the roof has a flexible body which is arranged immediately beneath the roof material having the crack. A water leakage prevention method to prevent water leakage from a portion having a crack in the roof material laminated and fixed to the roof comprises steps of detaching the roof material at the portion having the crack, arranging the water leakage prevention material immediately beneath the roof material having the crack, and fixing the detached roof material back to the original position.