A method of designing a welded structure, the method comprising: a test piece preparation step of preparing a test piece including a welding structure in which a welding material formed of an austenitic alloy is welded to a member formed of low-alloy steel or low-carbon steel; a hydrogen supply step of supplying hydrogen to the test piece; a characteristic stress acquisition step of applying a load to the test piece to which hydrogen was supplied and acquiring the characteristic stress showing material mechanical properties of the test piece, and a design step of designing the welded structure including the welding structure, the design step including obtaining, by stress analysis, maximum stress occurring in the welding structure and, if the maximum stress is greater than or equal to the characteristic stress acquired in the characteristic stress acquisition step, redesigning the welded structure, wherein the welded structure includes a first member formed of the low-alloy steel or the low-carbon steel, a first welding member formed of a first welding material serving as the welding material built-up welded to the surface of the first member, and a second member welded to the surface of the first welding member via a second welding material, wherein the first member has a recessed portion and a side surface of the recessed portion being inclined gradually toward an outer side of the recessed portion as the side surface of the recessed portion approaches a surface around the recessed portion, wherein the surface of the first member for built-up welding the first welding material is a bottom surface and a side surface of the recessed portion, and wherein, in the design step, said redesigning includes changing a region of a side surface of the recessed portion in a first direction parallel with a surface extending around the recessed portion and away from a position in which a surface of a second welding member formed of the second welding material is in contact with a surface of the first welding member, wherein the hydrogen supply step is continuously executed while the characteristic stress acquisition step is executed, from a time before the execution of the characteristic stress acquisition step. a test piece preparation step of preparing a test piece including a welding structure in which a welding material formed of an austenitic alloy is welded to a member formed of low-alloy steel or low-carbon steel; a hydrogen supply step of supplying hydrogen to the test piece; a characteristic stress acquisition step of applying a load to the test piece to which hydrogen was supplied and acquiring the characteristic stress showing material mechanical properties of the test piece, and a design step of designing the welded structure including the welding structure, the design step including obtaining, by stress analysis, maximum stress occurring in the welding structure and, if the maximum stress is greater than or equal to the characteristic stress acquired in the characteristic stress acquisition step, redesigning the welded structure, wherein the welded structure includes a first member formed of the low-alloy steel or the low-carbon steel, a first welding member formed of a first welding material serving as the welding material built-up welded to the surface of the first member, and a second member welded to the surface of the first welding member via a second welding material, wherein the first member has a recessed portion and a side surface of the recessed portion being inclined gradually toward an outer side of the recessed portion as the side surface of the recessed portion approaches a surface around the recessed portion, wherein the surface of the first member for built-up welding the first welding material is a bottom surface and a side surface of the recessed portion, and wherein, in the design step, said redesigning includes changing a region of a side surface of the recessed portion in a first direction parallel with a surface extending around the recessed portion and away from a position in which a surface of a second welding member formed of the second welding material is in contact with a surface of the first welding member, wherein the first member has a recessed portion and a side surface of the recessed portion being inclined gradually toward an outer side of the recessed portion as the side surface of the recessed portion approaches a surface around the recessed portion, wherein the surface of the first member for built-up welding the first welding material is a bottom surface and a side surface of the recessed portion, and wherein, in the design step, said redesigning includes changing a region of a side surface of the recessed portion in a first direction parallel with a surface extending around the recessed portion and away from a position in which a surface of a second welding member formed of the second welding material is in contact with a surface of the first welding member, wherein the hydrogen supply step is continuously executed while the characteristic stress acquisition step is executed, from a time before the execution of the characteristic stress acquisition step.