A coil device 10 comprises a first bobbin 40 having a first bobbin plate 42 provided with a first hollow cylinder 44 on which a primary coil 20 is wound at the outer periphery, and a second bobbin 50 mounted on the first bobbin 40 and having a second bobbin plate 52 provided with a second hollow cylinder 54 on which a secondary coil 30 is wound at the outer periphery. A winding center C1 of the primary coil 20 and a winding center C2 of the secondary coil 30 displace with a predetermined displacement (Lx) along a predetermined reference direction X.