To solve problems wherein the number of parts increases when a yoke and a sleeve are separate parts and burr caused during manufacture is viscous because a soft iron material is used when the yoke and the sleeve are integrally provided, thereby making it difficult to remove burr.

First, the sleeve 11 and the yoke 18 are integrally formed by the soft iron material, and then surface hardening treatment is applied. In this hardening treatment process, viscous burr is hardened and becomes fragile. In the following deburring process, it is enough to remove non-viscous fragile burr, which can be easily removed. The sleeve 11 and the yoke 18 are constituted by one integral part to reduce the number of parts, and burr which becomes a problem when integrating them can be easily removed. Since a surface of the sleeve 11 is hardened, durability of an oil flow control valve 2 can be increased.