A gap measurement device (20;40;50) that is configured to measure a gap between an upper plate and a lower plate in a material formed of the upper plate and the lower plate that overlap with each other in a thickness direction, the device (20;40;50) comprising: a plate thickness measurement sensor that is configured to measure a plate thickness, the plate thickness being a thickness of the upper plate; a step measurement sensor that is configured to measure a step, the step being a distance between an upper surface of the upper plate and an upper surface of the lower plate; a calculation unit that calculates the gap between the upper plate and the lower plate by subtracting the plate thickness from the step; a displacement sensor that is configured to measure a first displacement (ΔZ1), the first displacement (ΔZ1) being a displacement, in the thickness direction, of a support portion supporting the plate thickness measurement sensor in a vertical direction; and an attitude determination unit that is configured to determine an attitude of the plate thickness measurement sensor facing the upper surface of the upper plate, wherein the step measurement sensor is configured to measure a second displacement (ΔZ2) and a first attitude angle (θ), the second displacement (ΔZ2) being a displacement of the step measurement sensor in the thickness direction, and the first attitude angle (θ) being an attitude angle of the step measurement sensor facing the upper surface of the upper plate, the calculation unit is configured to calculate a second attitude angle (φ), based on a distance between the plate thickness measurement sensor and the step measurement sensor at each of measurement points, the first displacement (ΔZ1), and the second displacement (ΔZ2), the second attitude angle (φ) being a component of an attitude angle of the plate thickness measurement sensor facing the upper surface of the upper plate, and the attitude determination unit is configured to determine the attitude, based on the first attitude angle (θ) and the second attitude angle (φ). a plate thickness measurement sensor that is configured to measure a plate thickness, the plate thickness being a thickness of the upper plate; a step measurement sensor that is configured to measure a step, the step being a distance between an upper surface of the upper plate and an upper surface of the lower plate; a calculation unit that calculates the gap between the upper plate and the lower plate by subtracting the plate thickness from the step; a displacement sensor that is configured to measure a first displacement (ΔZ1), the first displacement (ΔZ1) being a displacement, in the thickness direction, of a support portion supporting the plate thickness measurement sensor in a vertical direction; and an attitude determination unit that is configured to determine an attitude of the plate thickness measurement sensor facing the upper surface of the upper plate, wherein the step measurement sensor is configured to measure a second displacement (ΔZ2) and a first attitude angle (θ), the second displacement (ΔZ2) being a displacement of the step measurement sensor in the thickness direction, and the first attitude angle (θ) being an attitude angle of the step measurement sensor facing the upper surface of the upper plate, the calculation unit is configured to calculate a second attitude angle (φ), based on a distance between the plate thickness measurement sensor and the step measurement sensor at each of measurement points, the first displacement (ΔZ1), and the second displacement (ΔZ2), the second attitude angle (φ) being a component of an attitude angle of the plate thickness measurement sensor facing the upper surface of the upper plate, and the attitude determination unit is configured to determine the attitude, based on the first attitude angle (θ) and the second attitude angle (φ).