A surface shape measurement apparatus comprising: a stage arranged to support a measurement object; a measurement device arranged to measure measurement data being surface shape data of the measurement object by scanning said object in a contactless manner, said measurement device comprising an angle measurement device configured to measure inclination of a surface of the measurement object at a measurement point to provide an output value; a computation part configured to compute by ray tracing computation surface shape data of the measurement object based on said measurement data, including said output value, of the measurement device, predetermined reference shape data of the measurement object, and a sampling interval when scanning, assuming that the object is placed at a target placement position of the measurement object on the stage; and said computation part being further configured to calculate, based on information of the target placement position of the measurement object on the stage, the computed surface shape data and the reference shape data, a placement error between an actual placement position of the measurement object on the stage and the target placement position by comparing surface information of the predetermined reference shape data of the measurement object assuming that the measurement object is placed at a target placement position of the measurement object on the stage and the surface shape data that is computed by the computation part, the comparing being performed by fitting the surface shape of the predetermined reference shape data with surface shape data that is computed by the computation part, whereby the computation part is configured to compute by ray tracing computation updated surface shape data of the measurement object assuming that the measurement object is placed at a placement position of the measurement object on the stage which is deviated from the target placement position by the placement error to update surface shape data of the measurement object using the placement error. a stage arranged to support a measurement object; a measurement device arranged to measure measurement data being surface shape data of the measurement object by scanning said object in a contactless manner, said measurement device comprising an angle measurement device configured to measure inclination of a surface of the measurement object at a measurement point to provide an output value; a computation part configured to compute by ray tracing computation surface shape data of the measurement object based on said measurement data, including said output value, of the measurement device, predetermined reference shape data of the measurement object, and a sampling interval when scanning, assuming that the object is placed at a target placement position of the measurement object on the stage; and said computation part being further configured to calculate, based on information of the target placement position of the measurement object on the stage, the computed surface shape data and the reference shape data, a placement error between an actual placement position of the measurement object on the stage and the target placement position by comparing surface information of the predetermined reference shape data of the measurement object assuming that the measurement object is placed at a target placement position of the measurement object on the stage and the surface shape data that is computed by the computation part, the comparing being performed by fitting the surface shape of the predetermined reference shape data with surface shape data that is computed by the computation part, whereby the computation part is configured to compute by ray tracing computation updated surface shape data of the measurement object assuming that the measurement object is placed at a placement position of the measurement object on the stage which is deviated from the target placement position by the placement error to update surface shape data of the measurement object using the placement error.