To produce a hollow core for a coaxial cable having high hollow rate and stable electric characteristics in its longitudinal direction. A producing method of a hollow core for a coaxial cable, the hollow core comprising: an inner conductor; and an insulating coating body including an inner annular portion for coating the inner conductor, a plurality of ribs extending radially from the inner annular portion, and an outer annular portion having an outer diameter of 0.5 mm or less for connecting outer ends of the ribs with each other; in which the hollow core includes a plurality of hollow portions surrounded by the inner annular portion, the outer annular portion and the ribs, a ratio of an area of the hollow portion in the insulating portion is 40% or more, and a roundness of the outer annular portion is 96.0% or more, wherein the producing method comprises at least: a step (1) of extruding molten resin from the die that capable of forming the insulating coating body; a step (2) of heating a resin forming the insulating coating body; and a step (3) of cooling the resin forming the insulating coating body slowly at a temperature close to a room temperature.