Obtaining meaningful numerical data for circular disks having radii larger than 3.0 wavelengths using double precision or quadruple precision arithmetic has proven difficult. The present study investigates the reason for this difficulty and obtains numerical data for diffracted fields of disks of larger radii. In addition, the characteristics of the diffracted field are discussed. Calculation of the current distribution using the GNU Multiple Precision Arithmetic Library and the Multiple Precision Floating-Point Reliable Library are demonstrated to provide meaningful data for disks having radii larger than 3.5 wavelengths. Finally, the calculation precision of the coefficients of simultaneous linear equations is revealed to be the source of the difficulty in obtaining meaningful numerical data for circular disks of larger radii.