An expansion turbine (10A) includes: a turbine housing (13) including a scroll section for taking in working fluid (w); a plurality of variable nozzles (36) disposed inside the turbine housing at intervals in a circumferential direction of the expansion turbine, each of the variable nozzles being configured to be rotatable about a rotation shaft (38); and a turbine wheel (24) disposed rotatably inside the turbine housing, the turbine wheel including a plurality of turbine blades disposed downstream of the variable nozzles. The turbine housing has a first wall surface (50) including a shroud section which faces tips (28a) of the turbine blades, and a second wall surface (52) which faces the first wall surface across a flow path (f) of the working fluid. A blade height of the variable nozzles at an outlet side is greater than a blade height of the turbine blades at an inlet side. The first wall surface has a protruding portion (44) protruding toward the second wall surface so as to reduce a height of the flow path of the working fluid toward downstream, at a downstream side of an outlet of the variable nozzles and an upstream side of an inlet of the turbine blades, and a protruding amount of the protruding portion is set so that a tip end of the protruding portion does not protrude toward the second wall surface past the tips of the turbine blades at the inlet.