To provide an handleable gap measuring device capable of acquiring highly accurate measured value, regardless of the measurement location or the minute gap.

A measuring piece 4, having an inclined plane 4d inclined toward the tip at a fixed angle θ, for being inserted between a measuring object reference plane Na and a measuring object part Ma of an object M is shifted by a slider 2, until the inclined plane 4d butts the measuring object part Ma. The gap quantity Gx can be calculated, based on a shift quantity D of the measuring piece 4 from a measurement reference position S, the inclination angle θ of the inclined plane 4d or the like. Hereby, the gap quantity Gx can be measured easily and highly accurately by only inserting the measuring piece 4, even if the gap is too small or the location is difficult to make measurement at, using a caliper or the like.