An apparatus for launching an underwater payload with water pressure, the launching apparatus for underwater payloads comprising: a launch tube suitable for having an underwater payload loaded inside; and a conduit including a first end portion, which is one of axially opposite ends, and a communicating portion located in a position separate from the first end portion and communicating with the launch tube, the conduit being filled with water, a plurality of conduit coils ( 11 to 15) disposed around the conduit in a first region ( 4R1) between the communicating portion and the first end portion, the plurality of conduit coils aligned axially of the conduit; a plurality of main circuits ( 41 to 45) configured to pass an electric current through the conduit coils, respectively, so as to generate an axial first magnetic field in the conduit; a main-circuit control section configured to control an operation of each of the plurality of main circuits; and a moving object disposed in the conduit and including a magnetic material or an induction coil, the moving object being configured to move in the conduit in an advance direction from the first end portion toward the communicating portion to thereby generate, in the launch tube, water pressure for launching the underwater payload, wherein the main-circuit control section controls the operation of each of the plurality of main circuits such that the main circuits pass electric currents through the conduit coils corresponding to the main circuits, respectively, to generate the axial first magnetic field and to thereby cause an axial second magnetic field ( mb) in the moving object, the first magnetic field and the second magnetic field making an interaction which moves the moving object in the advance direction. a launch tube suitable for having an underwater payload loaded inside; and a conduit including a first end portion, which is one of axially opposite ends, and a communicating portion located in a position separate from the first end portion and communicating with the launch tube, the conduit being filled with water, a plurality of conduit coils ( 11 to 15) disposed around the conduit in a first region ( 4R1) between the communicating portion and the first end portion, the plurality of conduit coils aligned axially of the conduit; a plurality of main circuits ( 41 to 45) configured to pass an electric current through the conduit coils, respectively, so as to generate an axial first magnetic field in the conduit; a main-circuit control section configured to control an operation of each of the plurality of main circuits; and a moving object disposed in the conduit and including a magnetic material or an induction coil, the moving object being configured to move in the conduit in an advance direction from the first end portion toward the communicating portion to thereby generate, in the launch tube, water pressure for launching the underwater payload, wherein the main-circuit control section controls the operation of each of the plurality of main circuits such that the main circuits pass electric currents through the conduit coils corresponding to the main circuits, respectively, to generate the axial first magnetic field and to thereby cause an axial second magnetic field ( mb) in the moving object, the first magnetic field and the second magnetic field making an interaction which moves the moving object in the advance direction.