In a magnetic reading/writing device, a controller derives at each radius of a recording medium a slope of a curve of track-averaged write performance with respect to an adjustment parameter, and determines a first set of fitting coefficients of a first equation approximating the derived slopes in terms of a first variable representing each radius. The controller acquires write performance dependence on a variable representing each of multiple circumferential positions of the medium by measuring track average write performance with respect to the circumferential positions, and determines a second set of fitting coefficients to approximate by a periodic function the acquired dependence in terms of the first variable. The controller corrects a condition value representing the adjustment parameter by subtracting from the condition value an adjustment value obtained from functions calculated with the first and second sets of fitting coefficients.