Abstract: In topology optimization (TO) of a structure, a mechanical field of the structure is required to evaluate the objective function and/or constraints. In a model-order-reduction method for efficiently computing the mechanical field, a fine-scale structure modelling the structure is first coarsened to give a coarse-scale structure. A finite element method (FEM) is applied to the coarse-scale structure to obtain a coarse-scale mechanical field. A fine-scale mechanical field is computed from the coarse-scale one instead of using the FEM to directly compute the fine-scale mechanical field from the fine-scale structure, allowing the fine-scale mechanical field with a higher accuracy than the coarse-scale one to be used as the mechanical field while achieving computation cost saving. In generating the fine-scale mechanical field, an artificial neural network, entitled as MapNet, is used to map the coarse-scale mechanical field to the fine-scale one.