1. A method performed by a processor for verifying model-design which verifies design of a 3D model, the method comprising: preparing a 3D model in a storage unit, the 3D model including a male-screw model and a tapped-hole model corresponding to the male-screw model, the male-screw model having a first diameter, the tapped-hole model having a second diameter that is a prepared-hole diameter, and the tapped hole model having an attribute that is used as a screw diameter for the second diameter; checking at a checking unit a relationship between the first diameter of the male-screw model and the second diameter of the tapped-hole model in a first case where the second diameter is the prepared-hole diameter to obtain the relationship indicating interference between the male-screw model and the tapped-hole model based on the checking for the first case; changing at a changing unit the second diameter of the tapped-hole model to the screw diameter from the prepared-hole diameter, when the obtained relationship that is a result of the checking on the first case indicates interference; checking at a checking unit a relationship between the first diameter of the male-screw model and the second diameter of the tapped-hole model in a second case where the second diameter is the screw diameter to obtain the relationship indicating interference or contact between the male-screw model and the tapped-hole model based on the checking for the second case and the changing; verifying at a model-fit verification unit fit between the male-screw model and the tapped-hole model based on a result of the checking for the first case and the checking for the second case; and outputting a result of the model-fit verification from an output unit, wherein the model-fit verification unit determines that the male-screw model and the tapped-hole model fit with each other when the obtained relationship that is a result of the checking in the first case indicates interference, and when the relationship that is a result of the checking in the second case indicates contact, and wherein the output unit outputs the result of the model-fit verification indicating that the relationship between the male screw and tapped hole is not normal when the model-fit verification unit determines that the male-screw model and the tapped-hole model does not fit with each other.