A molten metal pump, which comprises a pump chamber portion and a drive unit and is configured to drive molten metal in a pump chamber of the pump chamber portion by the drive unit so as to discharge the molten metal through a pump chamber outlet of the pump chamber portion and suck external molten metal through a pump chamber inlet of the pump chamber portion into the pump chamber, wherein the pump chamber portion has an outer cylinder and an inner cylinder detachably housed in the outer cylinder, the outer cylinder being configured as a bottomed cylindrical body having an outer cylinder bottom wall and an outer cylinder side wall, the inner cylinder being configured as a bottomed cylindrical body having an inner cylinder bottom wall and an inner cylinder side wall, a ring-shaped spacer, which is interposed between an inner surface of the outer cylinder bottom wall and an outer surface of the inner cylinder bottom wall in a sealed state and in a detachable manner, is further provided, the spacer partitioning a space between an inner surface of the outer cylinder bottom wall and an outer surface of the inner cylinder bottom wall to form the pump chamber, the pump chamber communicating with outside through the pump chamber inlet formed at the outer cylinder bottom wall and the pump chamber outlet formed at the outer cylinder side wall, the drive unit has a rotary magnet provided inside the inner cylinder so as to be rotatable about a vertical rotation axis, the rotary magnet having a plurality of permanent magnets arranged on a circumference around the rotation axis, the plurality of permanent magnets each having an upper surface magnetic pole and a lower surface magnetic pole magnetized so that an upper surface part and a lower surface part become magnetic poles, the plurality of permanent magnets are arranged so that upper surface magnetic poles and lower surface magnetic poles having different magnetic poles are alternately aligned along the circumference, the plurality of lower surface magnetic poles of the plurality of permanent magnets vertically facing the inner cylinder bottom wall, and strengths of the plurality of permanent magnets are set so that a line of magnetic force from a first permanent magnet penetrates the inner cylinder bottom wall downward to reach the pump chamber, and penetrates the inner cylinder bottom wall upward from the pump chamber to return to a second permanent magnet adjacent to the first permanent magnet. the pump chamber portion has an outer cylinder and an inner cylinder detachably housed in the outer cylinder, the outer cylinder being configured as a bottomed cylindrical body having an outer cylinder bottom wall and an outer cylinder side wall, the inner cylinder being configured as a bottomed cylindrical body having an inner cylinder bottom wall and an inner cylinder side wall, a ring-shaped spacer, which is interposed between an inner surface of the outer cylinder bottom wall and an outer surface of the inner cylinder bottom wall in a sealed state and in a detachable manner, is further provided, the spacer partitioning a space between an inner surface of the outer cylinder bottom wall and an outer surface of the inner cylinder bottom wall to form the pump chamber, the pump chamber communicating with outside through the pump chamber inlet formed at the outer cylinder bottom wall and the pump chamber outlet formed at the outer cylinder side wall, the drive unit has a rotary magnet provided inside the inner cylinder so as to be rotatable about a vertical rotation axis, the rotary magnet having a plurality of permanent magnets arranged on a circumference around the rotation axis, the plurality of permanent magnets each having an upper surface magnetic pole and a lower surface magnetic pole magnetized so that an upper surface part and a lower surface part become magnetic poles, the plurality of permanent magnets are arranged so that upper surface magnetic poles and lower surface magnetic poles having different magnetic poles are alternately aligned along the circumference, the plurality of lower surface magnetic poles of the plurality of permanent magnets vertically facing the inner cylinder bottom wall, and strengths of the plurality of permanent magnets are set so that a line of magnetic force from a first permanent magnet penetrates the inner cylinder bottom wall downward to reach the pump chamber, and penetrates the inner cylinder bottom wall upward from the pump chamber to return to a second permanent magnet adjacent to the first permanent magnet.