Dimension assurance of mask using plurality of types of pattern ambient environment

According to a mask verifying method of the embodiment, a difference between an actual dimension of a mask pattern and a simulation dimension is calculated as a computational estimated value. Moreover, a difference between an actual dimension of the mask pattern that is actually measured and a dimension on pattern data is calculated as an actually-measured difference. Then, it is verified whether a mask pattern dimension passes or fails based on the calculated value. When calculating the computational estimated value, a model function, which is set based on each correspondence relationship between an actual dimension and a mask simulation dimension of a test pattern, which includes a plurality of types of pattern ambient environments, to the mask pattern.