A distance measuring system includes: a reference member configured to be provided on a surface of a first pipe made of metal, the reference member serving as a reference for distance measurement; an attachment member provided on a surface of a second pipe, made of metal, connected with the first pipe through a weld; a distance sensor configured to be attached to the attachment member, to measure a distance to the reference member; and a measuring unit configured to measure the distance based on an output from the distance sensor.