A manufacturing device of a vehicle body component for manufacturing a vehicle body component, said vehicle body component comprising a component main body as a part of a vehicle body, the component main body being formed of a metal plate and having a bent portion with a thickened portion where a plate thickness of the metal plate is gradually increased toward a center side in a bent direction of the bent portion, the manufacturing device comprising: a first die and a second die, that are configured to hold between them a metal plate formed with a bent portion, the metal plate having a constant thickness, whereby the first die is configured to contact the metal plate from an outer side in a bent radial direction of the bent portion while the second die is configured to contact the metal plate from an inner side in the bent radial direction of the bent portion and a first gap is formed between the first die and the bent portion, a cross section of the first gap being gradually increased toward a center side in a bent direction of the bent portion; a movable die that is configured to pressurize one side portion of the metal plate to the bent portion side in a state that the metal plate is held between the first die and the second die, the one side portion of the metal plate being adjacent to the bent portion;and a device main body that is configured to use the movable die to pressurize the one side portion of the metal plate to the bent portion side in the state that the metal plate is held between the first die and the second die, so as to move a portion of the metal plate to the first gap by plastic flow, characterized in that the second die includes a slide portion that can increase or reduce a width dimension of a second gap between the second die and the bent portion of the metal plate having a constant thickness, along the bent radial direction of the bent portion, a cross-section of the second gap being gradually increased toward a center side in a bent direction of the bent portion, the device main body includes a load sensor for detecting a reactive force that the movable die receives during pressurization of the one side portion of the metal plate, and the movable die is configured to start pressurizing the one side portion of the metal plate in a state where the width dimension of the second gap is reduced by the slide portion, and the slide portion is configured to be moved so as to increase the width dimension of the second gap when output of the load sensor exceeds a predetermined threshold, the movable die being configured to continue pressurizing the one side portion of the metal plate as the slide portion is moved so as to increase the width dimension of the second gap so as to move a portion of the metal plate to the second gap by plastic flow. a first die and a second die, that are configured to hold between them a metal plate formed with a bent portion, the metal plate having a constant thickness, whereby the first die is configured to contact the metal plate from an outer side in a bent radial direction of the bent portion while the second die is configured to contact the metal plate from an inner side in the bent radial direction of the bent portion and a first gap is formed between the first die and the bent portion, a cross section of the first gap being gradually increased toward a center side in a bent direction of the bent portion; a movable die that is configured to pressurize one side portion of the metal plate to the bent portion side in a state that the metal plate is held between the first die and the second die, the one side portion of the metal plate being adjacent to the bent portion;and a device main body that is configured to use the movable die to pressurize the one side portion of the metal plate to the bent portion side in the state that the metal plate is held between the first die and the second die, so as to move a portion of the metal plate to the first gap by plastic flow, characterized in that the second die includes a slide portion that can increase or reduce a width dimension of a second gap between the second die and the bent portion of the metal plate having a constant thickness, along the bent radial direction of the bent portion, a cross-section of the second gap being gradually increased toward a center side in a bent direction of the bent portion, the device main body includes a load sensor for detecting a reactive force that the movable die receives during pressurization of the one side portion of the metal plate, and the movable die is configured to start pressurizing the one side portion of the metal plate in a state where the width dimension of the second gap is reduced by the slide portion, and the slide portion is configured to be moved so as to increase the width dimension of the second gap when output of the load sensor exceeds a predetermined threshold, the movable die being configured to continue pressurizing the one side portion of the metal plate as the slide portion is moved so as to increase the width dimension of the second gap so as to move a portion of the metal plate to the second gap by plastic flow.