A bending press device comprising: a punch; two linear dies which are arranged parallel to each other with a predetermined space therebetween, wherein the punch is adapted to press down a portion of a sheet-like material to be formed which is mounted on the two dies and is disposed above the space to form an open seam pipe having open seam edges (31a, 31b) which face each other in an opposed manner; a punch support portion which is adapted to support the punch, the punch support portion having an upper portion arranged to be positioned outside the open seam pipe and the punch support portion arranged to extend through the open seam edges (31a, 31b) to a lower portion positioned inside the open seam pipe at the time of pressing, the punch support portion being movable together with the punch in the direction that the material to be formed is pressed down or in the direction that the pressing-down is released; a measurement device; and a control part which is arranged to control a punch pressing-down amount based on a measurement by the measurement device; characterised in that: the measurement device is an open seam edge measurement device which includes a light projector and a light receiver mounted on the punch support portion, a first one of the light projector and the light receiver being mounted on the upper portion of the punch support portion while the other one is mounted on the lower portion of the punch support portion at a position facing the first one to form an optical path, and the measurement device adapted to measure an amount of gap (a 1, a 2) between the punch support portion and the open seam edge (31a, 31b) based on a change in intensity of a measurement light irradiated from the light projector to pass through the optical path and to reach the light receiver, the change being caused when the optical path is blocked by the open seam edge (31a, 31b) of the open seam pipe; and the control part is adapted to control a punch pressing-down amount based on the amount of gap (a 1, a 2) measured by the open seam edge measurement device. a punch; two linear dies which are arranged parallel to each other with a predetermined space therebetween, wherein the punch is adapted to press down a portion of a sheet-like material to be formed which is mounted on the two dies and is disposed above the space to form an open seam pipe having open seam edges (31a, 31b) which face each other in an opposed manner; a punch support portion which is adapted to support the punch, the punch support portion having an upper portion arranged to be positioned outside the open seam pipe and the punch support portion arranged to extend through the open seam edges (31a, 31b) to a lower portion positioned inside the open seam pipe at the time of pressing, the punch support portion being movable together with the punch in the direction that the material to be formed is pressed down or in the direction that the pressing-down is released; a measurement device; and a control part which is arranged to control a punch pressing-down amount based on a measurement by the measurement device; characterised in that: the measurement device is an open seam edge measurement device which includes a light projector and a light receiver mounted on the punch support portion, a first one of the light projector and the light receiver being mounted on the upper portion of the punch support portion while the other one is mounted on the lower portion of the punch support portion at a position facing the first one to form an optical path, and the measurement device adapted to measure an amount of gap (a 1, a 2) between the punch support portion and the open seam edge (31a, 31b) based on a change in intensity of a measurement light irradiated from the light projector to pass through the optical path and to reach the light receiver, the change being caused when the optical path is blocked by the open seam edge (31a, 31b) of the open seam pipe; and the control part is adapted to control a punch pressing-down amount based on the amount of gap (a 1, a 2) measured by the open seam edge measurement device. the measurement device is an open seam edge measurement device which includes a light projector and a light receiver mounted on the punch support portion, a first one of the light projector and the light receiver being mounted on the upper portion of the punch support portion while the other one is mounted on the lower portion of the punch support portion at a position facing the first one to form an optical path, and the measurement device adapted to measure an amount of gap (a 1, a 2) between the punch support portion and the open seam edge (31a, 31b) based on a change in intensity of a measurement light irradiated from the light projector to pass through the optical path and to reach the light receiver, the change being caused when the optical path is blocked by the open seam edge (31a, 31b) of the open seam pipe; and the control part is adapted to control a punch pressing-down amount based on the amount of gap (a 1, a 2) measured by the open seam edge measurement device.