A steering gear mounting method characterized in that: a base plate, a steering gear foundation arranged on the base plate, and a steering gear arranged on the steering gear foundation are assembled in one combined body in advance, and at the same time, a deck seal, which will slidingly contact a rudderstock with water tightness, is mounted into a peripheral portion of a center hole formed in the base plate; this combined body is hung in such a condition that the base plate is loosely fitted into a penetrating hole having prescribed diameter formed on a steering gear deck, and a rudderstock is inserted into the center hole of the base plate, through a rudderstock penetrating portion of a stern hull, and the top end of the rudderstock is connected to the steering gear, and thus a gathering body composed of the steering gear, the steering gear foundation, the base plate, the deck seal and the rudderstock is formed; this gathering body is supported on the steering gear deck through stay members and adjusting screw bolts of the base plate in the condition that the base plate is loosely fitted into the penetrating hole of the steering gear deck, and under this condition, position of the gathering body at the part of the base plate is adjusted within the horizontal plane by making the axis of the gathering body align with the axis of a neck bearing provided on the stern hull at a rudderstock penetrating portion, and at the same time, inclination and position in the vertical direction at the part of the base plate are adjusted by making the tips of the adjusting screw bolts arranged on the stay members of the base plate touch to the steering gear deck, and turning the adjusting screw bolts, and the gathering body is so maintained that its axis aligns with the axis of the neck bearing arranged at the rudderstock penetrating portion of the stern hull, and its position in the upper and lower direction becomes proper; and this gathering body is mounted to the steering gear deck, with the outer peripheral portion of the base plate being welded to the peripheral portion of the penetrating hole of the steering gear deck, and then a rudder blade is fitted to the lower end of the rudderstock. a base plate, a steering gear foundation arranged on the base plate, and a steering gear arranged on the steering gear foundation are assembled in one combined body in advance, and at the same time, a deck seal, which will slidingly contact a rudderstock with water tightness, is mounted into a peripheral portion of a center hole formed in the base plate; this combined body is hung in such a condition that the base plate is loosely fitted into a penetrating hole having prescribed diameter formed on a steering gear deck, and a rudderstock is inserted into the center hole of the base plate, through a rudderstock penetrating portion of a stern hull, and the top end of the rudderstock is connected to the steering gear, and thus a gathering body composed of the steering gear, the steering gear foundation, the base plate, the deck seal and the rudderstock is formed; this gathering body is supported on the steering gear deck through stay members and adjusting screw bolts of the base plate in the condition that the base plate is loosely fitted into the penetrating hole of the steering gear deck, and under this condition, position of the gathering body at the part of the base plate is adjusted within the horizontal plane by making the axis of the gathering body align with the axis of a neck bearing provided on the stern hull at a rudderstock penetrating portion, and at the same time, inclination and position in the vertical direction at the part of the base plate are adjusted by making the tips of the adjusting screw bolts arranged on the stay members of the base plate touch to the steering gear deck, and turning the adjusting screw bolts, and the gathering body is so maintained that its axis aligns with the axis of the neck bearing arranged at the rudderstock penetrating portion of the stern hull, and its position in the upper and lower direction becomes proper; and this gathering body is mounted to the steering gear deck, with the outer peripheral portion of the base plate being welded to the peripheral portion of the penetrating hole of the steering gear deck, and then a rudder blade is fitted to the lower end of the rudderstock.