A non-destructive testing device, comprising: a magnetic flux generator having an excitation controller, and a detection processor, which are adapted to cause a magnetic flux density to be generated by applying a magnetic field to a measurement subject, said testing device further comprising a magnetic flux detection element, said detection processor being adapted to measure magnetic flux emitted from a plurality of locations on the measurement subject by way of the magnetic flux detection element, said detection processor further being adapted to calculate, after the magnetic field is discontinued, a plurality of time constants of transient change in magnetic flux, and to detect an internal structure of the measurement subject from a distribution state of the time constants, wherein the magnetic flux detection element comprises a plurality of microcoils arranged in one vertical line and one horizontal line in a cross shape, wherein the detection processor is adapted to arrange the magnetic flux detection element at a predetermined location on the measurement subject, to perform a first measurement by way of first microcoils of the plurality of microcoils arranged in the one vertical line and to perform a second measurement by way of second microcoils of the plurality of mircrocoils arranged in the one horizontal line at the same location as the predetermined location, wherein the detection processor is adapted to estimate a center location of a nugget formed inside of the measurement subject and/or diameter of the nugget, by calculating an intersection between: a perpendicular line passing through a central point of a connecting line of two points extracted, by acquiring measurement results from the first measurement, and extracting from the measurement results two points of the one vertical line that are the most distant and correspond to a contour line of the nugget; and a perpendicular line passing through a central point of a connecting line of two points extracted, by acquiring measurement results from the second measurement, and extracting from the measurement results two points of the one horizontal line that are the most distant and correspond to the contour line of the nugget; setting the intersection thus calculated as a center location, and calculating a diameter of a circle passing through the two points of the one vertical line or the two points of the one horizontal line, centering around the center location. wherein the magnetic flux detection element comprises a plurality of microcoils arranged in one vertical line and one horizontal line in a cross shape, wherein the detection processor is adapted to arrange the magnetic flux detection element at a predetermined location on the measurement subject, to perform a first measurement by way of first microcoils of the plurality of microcoils arranged in the one vertical line and to perform a second measurement by way of second microcoils of the plurality of mircrocoils arranged in the one horizontal line at the same location as the predetermined location, wherein the detection processor is adapted to estimate a center location of a nugget formed inside of the measurement subject and/or diameter of the nugget, by calculating an intersection between: a perpendicular line passing through a central point of a connecting line of two points extracted, by acquiring measurement results from the first measurement, and extracting from the measurement results two points of the one vertical line that are the most distant and correspond to a contour line of the nugget; and a perpendicular line passing through a central point of a connecting line of two points extracted, by acquiring measurement results from the second measurement, and extracting from the measurement results two points of the one horizontal line that are the most distant and correspond to the contour line of the nugget; setting the intersection thus calculated as a center location, and calculating a diameter of a circle passing through the two points of the one vertical line or the two points of the one horizontal line, centering around the center location.