A magnetic disk driving device includes a frame which supports a shaft, a hub adapted to hold a disk, and upper and lower bearings which are fitted in a bearing holding hole and rotatably support the hub. The frame includes a cylindrical protrusion which is confronted through a small gap d with the outer cylindrical surface of the outer race of the lower bearing. The small gap d forms a labyrinth in the space through which the inside of the lower bearing is communicated with the outside. The inner cylindrical surface of an annular protrusion formed on the bottom of the hub, and the inner cylindrical surface of the cylindrical protrusion are substantially equal in diameter.