To provide a guarantee method of a building by which the building is restored while suppressing an economical burden of a purchaser of the building when the existing building or a building scheduled to be constructed is damaged by an earthquake.

In the guarantee method of the building for guaranteeing repair or reconstruction of the building when the existing building or the building scheduled to be constructed is damaged by the earthquake, the building where the purchaser of the building takes out an earthquake insurance is made into an assurance object and an assurer pays the balance between repair and reconstruction expenses (expenses for restoring to the status) required for repair or reconstruction of the building and an insurance amount paid based on the earthquake insurance which the purchaser takes out.