To provide a fundamental honeycomb structure reduced in residual strain at the time of molding and preventing the thickness of a wash coat layer from becoming different by the region of the sidewall of a small hole, its extrusion molding die and a manufacturing method of a large-sized honeycomb structure.

In the fundamental honeycomb structure 10 having square or hexagonal small holes 11 the same in cross-sectional shape arranged thereto side by side in an adjacent state, the insides of corner parts, where the adjacent sidewalls 12 of the small holes 11 cross each other, are smoothly connected by connection parts 13 of which the inner surfaces have an arcuate or linear cross-sectional shape and the cross-sectional width of each of the connection parts 13 is preferably 0.05-0.45 times the width of the sidewall on the short side in each of the sidewalls 12 to which the connection parts 13 are connected.