A method for indirect detecting of folding angles (.beta.) of a metal sheet (4) during the folding by a folding machine (1) which has a punch (2), and a die (3) provided with a depressed seat (5) is disclosed. The folding angle (.beta.) is obtained by algebraic summing of an angle (.alpha.) formed by adjacent two walls (10, 10, 13, 13, 13a, 13b) of the punch (2) or the seat (5) and each angle (.alpha.1, .alpha.2) between a portion (7, 8, 8a) of the metal sheet (4) and the said wall. The angles (.alpha.1, .alpha.2) formed between the said portion (7, 8, 8a) and each wall (10, 13, 13a, 13b) are calculated by using the measurements of the distances (D1, D2, D3, D4) from the wall to the portion measured at desired two points on the wall. The distances are detected by a device such as differential transformers (18, 19, 20) and pneumatic gauges (50, 51, 52, 53, 54, 55) incorporated in the punch (2) or the die (3). The angle (.beta., .beta.1, .beta.2) once achieved is calculated during the folding operation after elastic restoring of the metal sheet has been caused. If the angle calculated is less than the desired, a supplementary folding is carried out to achieve a precise folding angle. The whole folding is carried out by a single folding operation without requiring a second positioning of the metal sheet.