A honeycomb segment-forming die includes: an introducing portion provided on an introduction face side and having a plurality of back holes, and a forming portion provided on an formation face side and having slits communicating with the back holes. The forming portion has a polygonal outer peripheral shape and forms a prismatic honeycomb segment by passing kneaded clay for forming which is introduced from the back holes of the introducing portion through the slits of the forming portion, and a gap between slits formed in each of corner portions including a vertex of the polygonal forming portion is larger than a gap of slits formed in the central portion of the forming portion. This enables to form a honeycomb segment having high compressive strength.