A coil device includes a coil body with a wiring coil wound therearound. The coil body is composed of an insulative core cover with a core received therein. An insulative base is provided with terminals, which the coil body is to be mounted thereon. The core cover includes a planar support portion to be attached to the base, and one of the support portion and the base has, at areas in the vicinity of opposite end portions thereof, engagement grooves, and the other has, at areas in the vicinity of opposite ends thereof, ridges engageable with the engagement grooves. Thus, the support portion can be engaged with the base through a combination of the engagement grooves and the ridges.