The present invention relates to a method for quantitatively measuring the muscular relaxing activity of a neurotoxin. Specifically, based on an extent of the activity of muscular relaxation of a neurotoxin from bacteria of Clostridium, the present invention relates to a method for quantification of the efficacy (potential and/or diffusion reaction) of a neurotoxin comprising the following steps of: (a) administering a neurotoxin to the hind leg muscle of one of hind legs of a non-human mammal; (b) applying electric stimulus to said non-human mammal; (c) measuring a compound muscle action potential (CMAP) by contraction of said hind leg muscle to which the neurotoxin is administered and/or of the hind leg muscle of the other hind leg to which the neurotoxin is not administered; and (d) taking amplitude data from the compound muscle action potential (CMAP) obtained by the measurement in step (c) and analyzing an extent of a decrease in amplitude to thereby quantify the efficacy of the muscular relaxing activity by the neurotoxin. In contrast to the mouse LD50 currently used as a potential unit of a botulinum toxin which is measurable at a level of only several units, the quantification method of the efficacy of a neurotoxin of the present invention allows for measurement at a level of as low as 0.01 to 1 unit and hence is a method with a high sensitivity, reproducibility and accuracy.