PURPOSE: To prevent winding of a coil from bein disturbed.

CONSTITUTION: A coil bobbin comprises a trunk part 11 having a substantially square shaped external periphery cross section on which a coil is wound. a side wall part 12 for restricting a range where the coil is wound, a terminal 13 provided on the lower end of the side wall part 12, and a guide groove 15 formed in the internal surface of the side wall part 12 so as to make contact with the side surface of the trunk 11 for guiding a winding starting part of the coil and a winding final part. The guide groove 15 passes through a first corner part 16 located on the lower side of the trunk part 11 and permits its tip end to reach an upper side second corner part 17 and further has its depth at the first corner part 16 made to be deeper than the diameter of the coil. Further, the guide groove 15 is progressively made shallower from the starting point to a final point side of the second corner part 17 side of the trunk part 1.