A detection device and a detection method for automatically detecting flaws occurring on a raw tube manufactured by rolling a hollow shell by using a mandrel mill. The flaw detection device (100) comprises a wall thickness gauge (1) disposed on the extension side of the mandrel (M) and measuring the wall thicknesses of the hollow shell (P) in a press-down direction on the stands (#1 to #5) of themandrel, rolling load measuring devices (2) measuring rolling loads on the stands (#1 to #5), and an judgement device (3) for judging whether flaws are present or absent on the raw tube based on the measured wall thickness values of the hollow shell (P) in the press-down direction and the measured rolling load values on the stands. The judgement device (3) judges that flows occur on the raw tube when the measured wall thickness value of the hollow shell in the press-down direction on any of the stands (#1 to #5) partially varies a predetermined amount or more and the measured rolling load value of the hollow shell in the press-down direction on any of the stands (#1 to #5) partially varies a predetermined amount or more.