An apparatus is used to form a container from a blank which has innermost fold lines that confine a closed bottom of the container, and outer fold lines that extend outwardly from four corners of the bottom. The apparatus includes a blank forming device which defines a forming passage of polygonal cross-section and having four corners. A blank feeding device is adapted to feed the blank towards the blank forming device in a first direction substantially parallel to the cross-section of the forming passage. A punching mechanism has a punch head of polygonal cross-section and capable of passing through the forming passage in a second direction transverse to the first direction. The punch head has a forward end formed with a polygonal push pad for pushing the blank through the forming passage. The blank forming device has a pair of first folding parts and a pair of second folding parts which confine the forming passage and which are adapted to fold the blank about the innermost fold lines when the punch head pushes the blank through the forming passage in the second direction. The first and second folding parts are spaced apart from each other adjacent to the corners of the forming passage.
