A vane device includes a rotary shaft and vanes. A shroud unit includes a bottom plate section and a blocking plate section. The bottom plate section is disposed below the rotary shaft. The blocking plate section has a bottom end at the rear of the bottom plate section, and a top end disposed at the rear of the vanes in a spaced apart manner. A distance from the top end to the rotary shaft is 1.4 to 3 times the radial length of each vane. The blocking plate section prevents the vanes from rotating in a second direction opposite to a first direction when the vanes rotate in the first direction at a level below the top end of the blocking plate section.