PURPOSE: To reduce drop-out by obtaining constants which satisfy a fixed expression and forming a base for magnetic recording tape on a basis of these obtained constants.

CONSTITUTION: Information is recorded on a magnetic recording tape with an optimum recording current, and drop-out is counted under a fixed condition at a reproducing time, and the increment of drop-out is obtained by repeating this test. As the result of investigation of this test, expression (5.0A-586.7)A-1.04H1- (0.0027 H2-7.46)H2+16971.8 is true in this increment. A is the average crystal particle diameter (angstrom), and H is the number per 1mm3 of grain size having following ranges; H1=0.27 to 0.53μ and H2=0.54 to 0.80μ. Consequently, since it is sufficient if the increment of drop-out is 20 or less, A, H1 and H2 are determined to make the increment of 20 or less. Drop-out of the recording tape can be reduced by using the base obtained in this manner.