An inexpensive solenoid drive device with high magnetic performance is implemented in which a portion accommodating a plunger and a portion attracting the plunger are integrated with a core as a ferromagnetic body. A core includes a magnetic flux limiting portion that is located in a region on the biasing direction side of a first imaginary boundary plane and that is formed so as to have a small thickness in a radial direction, and an increased diameter portion that is formed so that its thickness the radial direction is increased in a continuous or stepwise manner from the first imaginary boundary plane toward the opposite biasing direction side. The increased diameter portion has a recessed portion whose outer surface is recessed toward a radial inner surface with respect to a predetermined increased diameter reference line in axial section of the core.