1. A manufacturing method of a photomask, the method comprising: setting an exposure condition applied when a resist is formed into a three-dimensional target shape by using a photomask including a plurality of light-shielding areas; setting a hypothetical target shape obtained by correcting the target shape based on a development characteristic of the resist for the exposure condition; creating a pattern of the photomask corresponding to the hypothetical target shape; simulating a prediction shape of the resist when the pattern is used; calculating a cost function related to an error between the prediction shape and the hypothetical target shape; and adjusting the pattern based on a result of the calculation of the cost function.