PURPOSE: To reduce a pressure drop of introduced gas in a contact process for producing sulfuric acid by specifying the shape and constitution of a catalyst in a catalyst bed.

CONSTITUTION: A converter packed with a single layer or plural layers of catalyst bed for producing sulfuric acid is used, wherein said catalyst bed(s) is(are) constituted of a central part contg. ladder or honeycomb catalysts causing parallel flow of gas and an external peripheral part contg, granular, annular or petaline catalysts. For example, a gas contg. SO2 and O2 is fed through a line 2 successively to a first catalyst bed (a) - fourth catalyst bed (d) packed in a converter 1. In the catalyst beds (a)-(d), plural ladder-shaped catalyst bed 20 causing parallel flow of gas are arranged to the central part of the beds, and annular catalyst beds 22 are arranged to a space between the catalyst beds 22 and a circular wall surface 21 of the converter. The gas discharged from the catalyst bed is introduced into a heat exchanger through a duct 3, 5 or 7, and fed back to the converter 1 through a duct 4, 6 or 8 after it is cooled. Thus, SO2 is converted to SO3.