Abstract: A wound core is formed by winding thread rubber (13) on a center (11) by holding the center (11) among three winding rollers (12), driving at least one winding roller to rotate the center, and feeding thread rubber (13) to the rotating center through a brake roller (16) and a delivery roller (14), thereby winding the thread rubber on the center under tension. The tension of thread rubber (13) is continuously measured upstream of the center by a tension gauge (42). The rotating condition of at least one of the winding roller (12), brake roller (16), and delivery roller (14) is adjusted in accordance with the tension measurement for adjusting the winding tension while winding the thread rubber on the center. When the tension of thread rubber is changed continuously or stepwise during winding, a wound core in which the hardness of the thread rubber layer varies continuously or stepwise in an thickness direction can be produced.