The invention relates to a steam generator (1) for a Rankine process, in particular for a waste-heat utilization device (37) of an internal combustion engine (36), preferably in a motor vehicle, comprising: a heat exchanger duct (2) in which a heat exchanger (3) is arranged, and a bypass duct (4) for bypassing the heat exchanger duct (2), wherein the heat exchanger duct (2) and the bypass duct (4) can, during operation of the steam generator (1), be traversed by a flow of a heating fluid, and wherein the heat exchanger (3) can, during operation of the steam generator (1), be traversed by a flow of a medium (47) to be evaporated. A compact design with high energy efficiency can be attained if the heat exchanger duct (2) encases the bypass duct (4).