A post holder for holding a safety post on a horizontal structure of a building is provided. The post holder has an elongate post reception portion an elongate central support portion, protruding from the post reception portion and perpendicular to the post reception portion, and first and second elongate side support portions protruding from the post reception portion laterally of the central support portion and in opposite directions. The central support portion comprises a fastening seat, which is arranged to receive a fastening member for fastening of the post holder to a base. Each one of said support portions comprises a support protrusion, which constitutes a fraction of the support portion at a bottom side thereof and protrudes at a bottom side thereof, thereby forming a lower most support surface of the support portion. The lower most support surfaces of the support portions are triangularly positioned.