To provide an undercut processing mechanism by which forming circle is shortened, assembly does not require much labor and time because constitution is simple, cost reduction is attained, the request of space saving is responsible because the mechanism is compactly constituted and, particularly, press working by which a formed goods are recessed to the insides facing to each other is simultaneously performed.

In the undercut processing mechanism in which a die 10 on a receiving side, which is inserted into a tubular base stock 1 on press working is made to be able to take out outside the tubular base stock 1 after performing the press working for recessing both wall surfaces of the tubular base stock 1 to the insides facing to each other, the die 10 on the receiving side is divided into three in the thickness direction which is orthogonally crossed with the axial direction of the tube in the opening on the one end side of the tubular base stock 1 and includes a pair of holding parts 11, 12 which are inserted from the opening on the one end side into the tubular base stock 1 and by which the portions 2 to be worked of both the wall surfaces are supported from the inside and an intermediate portion 13 which is put in between the respective holding parts 11, 12 and makes the respective holding parts 11, 12 approach and separate each other in the thickness direction.