A distance measuring method comprising: installing a reference member (300; 600) on a surface of a first metal member, and installing a mounting member (200; 200'; 500), to which a range finder (100; 400) is configured to be placed, on a surface of a second metal member, the second metal member being coupled to the first metal member via a weld, fitting first and second protruding parts (200A, 200B; 403A, 403B) into first and second fitting holes (103A, 103B; 500A, 500B), respectively, the first and the second protruding parts (200A, 200B; 403A, 403B) being included in one of the mounting member (200; 200'; 500) and the range finder (100; 400), the first and the second fitting holes (103A, 103B; 500A, 500B) being included in the other of the mounting member (200; 200'; 500) and the range finder (100; 400), inserting a first pressing pin through a first communicating hole (103S1) communicating with an inner side surface of the first fitting hole (103A; 500A), to press the first protruding part (200A; 403A) from a first direction toward an inner side surface of the first fitting hole (103A; 500A), and inserting a second pressing pin through a second communicating hole (103S2) communicating with an inner side surface of the second fitting hole (103B; 500B), to press the second protruding part (200B; 403B) from a second direction toward an inner side surface of the second fitting hole (103B; 500B), such that the range finder (100; 400) is placed to the mounting member (200; 200'; 500), the second direction being different from the first direction, and measuring a distance from the range finder (100; 400) to the reference member (300; 600), in a state where the range finder (100; 400) is placed to the mounting member (200; 200'; 500), wherein installing a reference member (300; 600) includes placing a plate member onto surfaces of the first and the second metal members (P1, P2) in such a manner as to extend across the weld so that the reference member (300; 600) and the mounting member (200; 200'; 500) including the first and the second protruding parts (200A, 200B; 403A, 403B) are installed, and the plate member includes a plurality of through holes into which the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) are configured to be fitted, the plurality of through holes being configured such that, when the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) are inserted into the through holes, the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) become in a predetermined positional relationship. installing a reference member (300; 600) on a surface of a first metal member, and installing a mounting member (200; 200'; 500), to which a range finder (100; 400) is configured to be placed, on a surface of a second metal member, the second metal member being coupled to the first metal member via a weld, fitting first and second protruding parts (200A, 200B; 403A, 403B) into first and second fitting holes (103A, 103B; 500A, 500B), respectively, the first and the second protruding parts (200A, 200B; 403A, 403B) being included in one of the mounting member (200; 200'; 500) and the range finder (100; 400), the first and the second fitting holes (103A, 103B; 500A, 500B) being included in the other of the mounting member (200; 200'; 500) and the range finder (100; 400), inserting a first pressing pin through a first communicating hole (103S1) communicating with an inner side surface of the first fitting hole (103A; 500A), to press the first protruding part (200A; 403A) from a first direction toward an inner side surface of the first fitting hole (103A; 500A), and inserting a second pressing pin through a second communicating hole (103S2) communicating with an inner side surface of the second fitting hole (103B; 500B), to press the second protruding part (200B; 403B) from a second direction toward an inner side surface of the second fitting hole (103B; 500B), such that the range finder (100; 400) is placed to the mounting member (200; 200'; 500), the second direction being different from the first direction, and measuring a distance from the range finder (100; 400) to the reference member (300; 600), in a state where the range finder (100; 400) is placed to the mounting member (200; 200'; 500), wherein installing a reference member (300; 600) includes placing a plate member onto surfaces of the first and the second metal members (P1, P2) in such a manner as to extend across the weld so that the reference member (300; 600) and the mounting member (200; 200'; 500) including the first and the second protruding parts (200A, 200B; 403A, 403B) are installed, and the plate member includes a plurality of through holes into which the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) are configured to be fitted, the plurality of through holes being configured such that, when the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) are inserted into the through holes, the reference member (300; 600) and the first and the second protruding parts (200A, 200B; 403A, 403B) become in a predetermined positional relationship.