Three‐dimensional codes for calculations of a stray magnetic field of iron core of a toroidal plasma confinement system have been developed. The calculated results agree well with the measured ones. It is found that up‐and‐down asymmetry of the iron‐core gap deforms the stray field asymmetrically when the primary coil current is not equal to the plasma current and that anisotropy of permeability of the iron core does not have a large influence on the field.