A magnetic disc unit includes a rotating disc type magnetic disc having an outer periphery, and a carriage arm linked to a head support mechanism for a recording/reproducing head. A base has an outer periphery and mounts the rotating disc type magnetic disc and the carriage arm, and has a side wall with an inner wall surface having an opening for inserting the carriage arm. The inner wall surface of the side wall defines a shroud surrounding the outer periphery of the magnetic disc, except at the opening in the side wall where the shroud is projected inwardly so as to decrease an opening angle of the opening in the side wall. A gap between the outer periphery of the magnetic disc and the shroud is set in a range not less than 0.1 mm but not greater than 0.6 mm.