A distance measuring device configured to measure, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, the distance measuring device comprising: a measuring member holding a length measuring instrument; at least one base member on which the measuring member is movably disposed; and a support leg that supports the base member and allows the base member to be detachably attached above the tube sheet, wherein a tip of the support leg on a lower end side enters an interior of a reaction tube beyond the opening of the reaction tube, a base end of the support leg on an upper end side has an outer diameter dimension larger than an inner diameter dimension of the opening of the reaction tube, the base member is attached to the reaction tube using the support leg inserted into the opening of the reaction tube, when a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle (α) formed by a straight line parallel to an axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line, a measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member, and the measuring member is disposed on the base member to be able to sequentially move from a position (P1) where the distance of one of the reaction tubes is measured to a position (P2) where the distance of another of the reaction tubes is to be measured. a measuring member holding a length measuring instrument; at least one base member on which the measuring member is movably disposed; and a support leg that supports the base member and allows the base member to be detachably attached above the tube sheet, wherein a tip of the support leg on a lower end side enters an interior of a reaction tube beyond the opening of the reaction tube, a base end of the support leg on an upper end side has an outer diameter dimension larger than an inner diameter dimension of the opening of the reaction tube, the base member is attached to the reaction tube using the support leg inserted into the opening of the reaction tube, when a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle (α) formed by a straight line parallel to an axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line, a measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member, and the measuring member is disposed on the base member to be able to sequentially move from a position (P1) where the distance of one of the reaction tubes is measured to a position (P2) where the distance of another of the reaction tubes is to be measured.