Turbomachine

A turbo machine comprising: an impeller having a plurality of blades therewith; a casing having a flow surface defined therein and being positioned with the impeller therein; and a plurality of grooves being formed in the flow surface of the casing, for connecting between an inlet side of said impeller and an area on the flow surface of the casing in which the blades of said impeller reside, on a periphery thereof, wherein an index of determining a form of the grooves is obtained by a following equation: JE No. = WR x VR x WDR x DLDR where, WR (a width ratio) is a value obtained by dividing a total value of the groove widths W with a length of casing periphery; VR (a volume ratio) is a value obtained by dividing a total volume of said grooves with a volume of said impeller; WDR (a width-depth ratio) is a value obtained by dividing the width W of said groove with a depth D of said groove; and DLDR is a ratio between a length of said groove in flow, being lower than the impeller inlet and the depth of said groove, and wherein, said grooves are formed so that the index JE No. lies in a range from 0.03 to 0.5.