An expansion turbine (10A, 10B) for recovering power from energy of a working fluid (w), comprising: a turbine housing (2a, 13) including a scroll section for taking in the working fluid (w); a plurality of variable nozzles disposed inside the turbine housing (2a, 13) at intervals in a circumferential direction of the expansion turbine (10A, 10B), each of the variable nozzles being configured to be rotatable about a rotation shaft so as to change an area of a flow path (f) of the working fluid (w) flowing from the scroll section; and a turbine wheel disposed rotatably inside the turbine housing (2a, 13), the turbine wheel including a plurality of turbine blades disposed downstream of the variable nozzles, wherein the turbine housing (2a, 13) includes an outer shell defining a profile of the turbine housing (2a, 13), a nozzle plate, and a nozzle mount disposed so as to face the nozzle plate, wherein the nozzle plate has a first wall surface including a shroud section which faces tips of the turbine blades, and the nozzle mount has a second wall surface which faces the first wall surface across the flow path (f) of the working fluid, wherein 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, wherein the shroud section has a protruding portion (44, 54) protruding toward the second wall surface so as to reduce a height of the flow path (f) of the working fluid (w) 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 wherein a protruding amount of the protruding portion (44, 54) is set so that a tip end of the protruding portion (44, 54) does not protrude toward the second wall surface past the tips of the turbine blades at the inlet, wherein the expansion turbine (10A, 10B) comprises a radial turbine configured such that the working fluid (w) flows into the inlet of the turbine blades along a radial direction of the expansion turbine (10A, 10B), and characterised in that where an origin is an axial position of roots of the turbine blades at the inlet and X t is an axial position of the tips of the turbine blades at the inlet of the turbine blades, an axial position X p of the tip end of the protruding portion (44, 54) satisfies a relationship 0 < X t ≤ X p. a turbine housing (2a, 13) including a scroll section for taking in the working fluid (w); a plurality of variable nozzles disposed inside the turbine housing (2a, 13) at intervals in a circumferential direction of the expansion turbine (10A, 10B), each of the variable nozzles being configured to be rotatable about a rotation shaft so as to change an area of a flow path (f) of the working fluid (w) flowing from the scroll section; and a turbine wheel disposed rotatably inside the turbine housing (2a, 13), the turbine wheel including a plurality of turbine blades disposed downstream of the variable nozzles, wherein the turbine housing (2a, 13) includes an outer shell defining a profile of the turbine housing (2a, 13), a nozzle plate, and a nozzle mount disposed so as to face the nozzle plate, wherein the nozzle plate has a first wall surface including a shroud section which faces tips of the turbine blades, and the nozzle mount has a second wall surface which faces the first wall surface across the flow path (f) of the working fluid, wherein 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, wherein the shroud section has a protruding portion (44, 54) protruding toward the second wall surface so as to reduce a height of the flow path (f) of the working fluid (w) 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 wherein a protruding amount of the protruding portion (44, 54) is set so that a tip end of the protruding portion (44, 54) does not protrude toward the second wall surface past the tips of the turbine blades at the inlet, wherein the expansion turbine (10A, 10B) comprises a radial turbine configured such that the working fluid (w) flows into the inlet of the turbine blades along a radial direction of the expansion turbine (10A, 10B), and characterised in that where an origin is an axial position of roots of the turbine blades at the inlet and X t is an axial position of the tips of the turbine blades at the inlet of the turbine blades, an axial position X p of the tip end of the protruding portion (44, 54) satisfies a relationship 0 < X t ≤ X p.