A magnetic head slider includes a pair of rails formed on a disk opposing surface opposed to a magnetic disk, each of the rails having a flat air bearing surface for generating a flying force during rotation of the disk, a slit formed between the pair of rails at their air inlet end portions, and a groove defined between the pair of rails which is continuous to the slit and wider than the slit, for expanding the air once compressed to generate a negative pressure. The magnetic head slider further includes an electromagnetic transducer formed on an air outlet end at a position where one of the rails is located. The slider has a trapezoidal shape such that its air inlet end is narrower than its air outlet end.