An impeller type pump has a pump body consisting of a front casing and a back casing. Each of the casings comprises a ceramic main portion and a protection member covering the main portion. The protection members of both the main portions are interconnected with each other at their confronting marginal portions through a continuous packing. Those confronting marginal portions of both the main portions which are adjacent to a pump chamber defined by inner surfaces of both the main portions are provided each with a plurality of concentrical rims loosely interdigitated to form a labyrinthine narrow passage restricting the flow of waste water therebetween, resulting in reduction of wear of the packing.