Embodiments of the invention provide a perpendicular magnetic writing head with a suppressed effective track width to be written on a magnetic medium while increasing writing magnetic field gradient. In one embodiment a trailing side shield is disposed by way of a gap film to a main pole of a perpendicular writing magnetic head. A gap distance (Gt) on a trailing side of the main pole and a gap distance (Gs) on a lateral side of the main pole is defined as Gt