A surface following system, comprising: a cooling water supplying part configured to supply cooling water to a surface of a moving object; a gas supply device; and a surface following nozzle connected to the gas supply device via a pipe and configured to inject gas supplied from the gas supply device toward the surface of the moving object, the surface following nozzle comprising: a nozzle configured to inject the gas from a tip end thereof; a separating part configured to close a base end of the nozzle; and an extending and contracting part that is provided at a rear side of the nozzle via the separating part, and configured to extend and contract along an axial direction of the nozzle, wherein the extending and contracting part includes an elastic body configured to apply a forward force with respect to the nozzle via the separating part, the surface following nozzle is configured to follow changes in the shape and changes in the distance of the moving object by a balance between a rearward force in the separating part that is generated by a pressure in the inside of the nozzle and the forward force that is generated by the elastic body, the system being configured such that when a spring constant of the elastic body is set as k [N/m], a necessary followed distance changing amount is set as x r [m], a maximum pressure of gas that can be supplied is set as P max [Pa], a minimum pressure of allowable gas is set as P min [Pa], a nozzle discharge area is set as S N [m 2], a maximum velocity of the cooling water in a vicinity of the tip end of the nozzle is set as V max, and a density of the cooling water is set as ρ W [kg/m 3], the spring constant k of the elastic body satisfies following formulas and, a contraction amount x[m] of the elastic body is within a range from x min to x max indicated by following formulas and, and the gas has a momentum that exceeds a momentum of the cooling water. k < P max − P min S N / x r P min = ρ W V max 2 / 2 x min = P min S N / k x max = P max S N / k a cooling water supplying part configured to supply cooling water to a surface of a moving object; a gas supply device; and a surface following nozzle connected to the gas supply device via a pipe and configured to inject gas supplied from the gas supply device toward the surface of the moving object, the surface following nozzle comprising: a nozzle configured to inject the gas from a tip end thereof; a separating part configured to close a base end of the nozzle; and an extending and contracting part that is provided at a rear side of the nozzle via the separating part, and configured to extend and contract along an axial direction of the nozzle, wherein the extending and contracting part includes an elastic body configured to apply a forward force with respect to the nozzle via the separating part, the surface following nozzle is configured to follow changes in the shape and changes in the distance of the moving object by a balance between a rearward force in the separating part that is generated by a pressure in the inside of the nozzle and the forward force that is generated by the elastic body, the system being configured such that when a spring constant of the elastic body is set as k [N/m], a necessary followed distance changing amount is set as x r [m], a maximum pressure of gas that can be supplied is set as P max [Pa], a minimum pressure of allowable gas is set as P min [Pa], a nozzle discharge area is set as S N [m 2], a maximum velocity of the cooling water in a vicinity of the tip end of the nozzle is set as V max, and a density of the cooling water is set as ρ W [kg/m 3], the spring constant k of the elastic body satisfies following formulas and, a contraction amount x[m] of the elastic body is within a range from x min to x max indicated by following formulas and, and the gas has a momentum that exceeds a momentum of the cooling water. k < P max − P min S N / x r P min = ρ W V max 2 / 2 x min = P min S N / k x max = P max S N / k a nozzle configured to inject the gas from a tip end thereof; a separating part configured to close a base end of the nozzle; and an extending and contracting part that is provided at a rear side of the nozzle via the separating part, and configured to extend and contract along an axial direction of the nozzle, wherein the extending and contracting part includes an elastic body configured to apply a forward force with respect to the nozzle via the separating part, the surface following nozzle is configured to follow changes in the shape and changes in the distance of the moving object by a balance between a rearward force in the separating part that is generated by a pressure in the inside of the nozzle and the forward force that is generated by the elastic body, the system being configured such that when a spring constant of the elastic body is set as k [N/m], a necessary followed distance changing amount is set as x r [m], a maximum pressure of gas that can be supplied is set as P max [Pa], a minimum pressure of allowable gas is set as P min [Pa], a nozzle discharge area is set as S N [m 2], a maximum velocity of the cooling water in a vicinity of the tip end of the nozzle is set as V max, and a density of the cooling water is set as ρ W [kg/m 3], the spring constant k of the elastic body satisfies following formulas and, a contraction amount x[m] of the elastic body is within a range from x min to x max indicated by following formulas and, and the gas has a momentum that exceeds a momentum of the cooling water. k < P max − P min S N / x r P min = ρ W V max 2 / 2 x min = P min S N / k x max = P max S N / k