An extrusion forming apparatus for forming a honeycomb formed body, the extrusion forming apparatus comprising: an extruding section which kneads a material including a ceramic raw material and extrudes a kneaded material from an extruding port; a chamber drum which includes: a first space portion having a first extruding direction extending portion connected to the extruding port and extending from the extruding port in an extruding direction, and a first outflow port opened in an end portion of a downstream side of the extruding direction, allowing the kneaded material to flow through the first space portion from the extruding port in the extruding direction and allowing the kneaded material to flow out from the first outflow port, and a second space portion having a second extruding direction extending portion connected to the first outflow port and extending from the first outflow port in the extruding direction, a direction changing portion (24d, 24d') bending from the extruding direction to a downward direction, and a second outflow port opened in the end portion of the downstream side, allowing the kneaded material to flow through the second space portion while changing a direction of the kneaded material from the extruding direction to the downward direction, and allowing the kneaded material to flow out from the second outflow port in the downward direction, wherein the second space portion has a cross section obtained by isotropically decreasing a cross section of the first space portion perpendicular to a flow-through direction of the kneaded material, wherein each of cross sections of a flow-through path formed by the second space portion is a cross section obtained by equally reducing the cross section of the flow-through path of the first space portion in all radial directions on the cross section; and a forming section connected to the second outflow port of the chamber drum, and having a die through which the kneaded material allowed to flow out from the second outflow port of the chamber drum passes inside, thereby a ceramic formed body is extruded. an extruding section which kneads a material including a ceramic raw material and extrudes a kneaded material from an extruding port; a chamber drum which includes: a first space portion having a first extruding direction extending portion connected to the extruding port and extending from the extruding port in an extruding direction, and a first outflow port opened in an end portion of a downstream side of the extruding direction, allowing the kneaded material to flow through the first space portion from the extruding port in the extruding direction and allowing the kneaded material to flow out from the first outflow port, and a second space portion having a second extruding direction extending portion connected to the first outflow port and extending from the first outflow port in the extruding direction, a direction changing portion (24d, 24d') bending from the extruding direction to a downward direction, and a second outflow port opened in the end portion of the downstream side, allowing the kneaded material to flow through the second space portion while changing a direction of the kneaded material from the extruding direction to the downward direction, and allowing the kneaded material to flow out from the second outflow port in the downward direction, a first space portion having a first extruding direction extending portion connected to the extruding port and extending from the extruding port in an extruding direction, and a first outflow port opened in an end portion of a downstream side of the extruding direction, allowing the kneaded material to flow through the first space portion from the extruding port in the extruding direction and allowing the kneaded material to flow out from the first outflow port, and a second space portion having a second extruding direction extending portion connected to the first outflow port and extending from the first outflow port in the extruding direction, a direction changing portion (24d, 24d') bending from the extruding direction to a downward direction, and a second outflow port opened in the end portion of the downstream side, allowing the kneaded material to flow through the second space portion while changing a direction of the kneaded material from the extruding direction to the downward direction, and allowing the kneaded material to flow out from the second outflow port in the downward direction, wherein the second space portion has a cross section obtained by isotropically decreasing a cross section of the first space portion perpendicular to a flow-through direction of the kneaded material, wherein each of cross sections of a flow-through path formed by the second space portion is a cross section obtained by equally reducing the cross section of the flow-through path of the first space portion in all radial directions on the cross section; and a forming section connected to the second outflow port of the chamber drum, and having a die through which the kneaded material allowed to flow out from the second outflow port of the chamber drum passes inside, thereby a ceramic formed body is extruded.