Method and device for evaluating charge potential

c h h c h c In a charge potential evaluation device, the measured value of a potential difference Vin a charged plate monitor (CPM) is converted into a potential difference Vbetween the conductive pattern and load beam in a head gimbal assembly (HGA), using the following expression (1), where ddenotes the distance between the conductive pattern and the load beam in the HGA, ddenotes the distance between the conductive plate and a grounded surface in the CPM, εdenotes the relative permittivity of an insulating foundation layer in the HGA, and εdenotes the relative permittivity of the region between the conductive plate and the grounded surface in the CPM.