1. A three dimensional shape measurement apparatus for measuring a 3D shape of an object existing on a measurement area, comprising: a pattern projection unit configured to project a pattern having a periodicity onto the measurement area; and a capturing unit configured to capture an image of the area onto which the pattern is projected, wherein the measurement area is specified by a reference plane, a projection area of the pattern projection unit, and a capturing area of the capturing unit, and the pattern projection unit projects the pattern to be focused on the reference plane, the apparatus further comprising: a phase information calculation unit configured to calculate phase information of a pattern of an image captured by the capturing unit when the object exists on the measurement area; a defocus amount calculation unit configured to calculate defocus amounts of the pattern in the captured image; and a three dimensional shape calculation unit configured to calculate a three dimensional shape of the object based on the phase information and the defocus amounts, and wherein the three dimensional shape calculation unit comprises a phase information correction unit configured to correct the phase information, and the phase information correction unit further configured to convert the defocus amounts into depth information of the measurement area from the capturing unit toward the reference plane, to calculate orders of phases corresponding to the depth information, and to correct the phase information based on the orders, and the three dimensional shape calculation unit further configured to calculate the three dimensional shape of the object based on the corrected phase information.