A sealing mechanism for a water pump comprising: a pump body; an impeller rotated by driving means; and a block which forms a vortex chamber together with the pump body, the vortex chamber housing the impeller, the sealing mechanism comprising an annular sealing member and respective attachment surfaces (14, 34) of the pump body and the block that surround an opening defined by the vortex chamber, wherein a first one of the attachment surfaces is provided with an annular groove, and wherein the pump body and the block are combined with each other via the respective attachment surfaces in a such manner that the annular sealing member is embedded in the annular groove and disposed between the pump body and the block, wherein the sealing mechanism further comprises: the annular sealing member including a protruding segment protruding from a periphery of the annular sealing member such that the protruding segment extends outward from an outer periphery of a second one of the attachment surfaces when the block and the pump body are combined via the respective attachment surfaces (14,34) with the sealing member embedded in the annular groove; characterised in that the annular groove includes a branching groove segment which branches out from the annular groove and is provided in said first one of the attachment surfaces such that the protruding segment is embedded in the branching groove segment when the block and the pump body are combined via the respective attachment surfaces (14,34) with the sealing member embedded in the annular groove, wherein said first one of the attachment surfaces has a single plane that extends continuously around the annular groove and the branching groove segment in a surrounding manner. a pump body; an impeller rotated by driving means; and a block which forms a vortex chamber together with the pump body, the vortex chamber housing the impeller, the sealing mechanism comprising an annular sealing member and respective attachment surfaces (14, 34) of the pump body and the block that surround an opening defined by the vortex chamber, wherein a first one of the attachment surfaces is provided with an annular groove, and wherein the pump body and the block are combined with each other via the respective attachment surfaces in a such manner that the annular sealing member is embedded in the annular groove and disposed between the pump body and the block, the annular sealing member including a protruding segment protruding from a periphery of the annular sealing member such that the protruding segment extends outward from an outer periphery of a second one of the attachment surfaces when the block and the pump body are combined via the respective attachment surfaces (14,34) with the sealing member embedded in the annular groove; characterised in that the annular groove includes a branching groove segment which branches out from the annular groove and is provided in said first one of the attachment surfaces such that the protruding segment is embedded in the branching groove segment when the block and the pump body are combined via the respective attachment surfaces (14,34) with the sealing member embedded in the annular groove, wherein said first one of the attachment surfaces has a single plane that extends continuously around the annular groove and the branching groove segment in a surrounding manner.