A method of calculating a current correction formula, comprising: a first measuring step (S106) to measure first voltage values at each of a plurality of current measuring parts (40A-40D) configured to measure currents in a plurality of socket parts (10A-10D) of a power strip in a state in which no current flows in the plurality of socket parts of the power strip; a second measuring step (S108-S114) to measure second voltage values at each of the plurality of current measuring parts in a state in which a current flows in one of the plurality of socket parts; a correction formula calculating step (S116) to calculate a correction formula formed by a inverse matrix of a matrix having as its elements differences between the second voltage values and the first voltage values, characterized in that there are provided: a third measuring step (S604-S610) to measure third voltage values output from each of a plurality of current measuring parts in a state in which one or more electrical apparatuses are electrically connected to the power strip; and an initial value calculating step (S118) to calculate initial values from differences between the third voltage values and the first voltage values, wherein the first measuring step, the second measuring step, the third measuring step, and the correction formula calculating step are performed for each of a plurality of different temperatures of the power strip, and wherein the initial values and the correction formula are used to calculate currents flowing in the plurality of socket parts of the power strip. a first measuring step (S106) to measure first voltage values at each of a plurality of current measuring parts (40A-40D) configured to measure currents in a plurality of socket parts (10A-10D) of a power strip in a state in which no current flows in the plurality of socket parts of the power strip; a second measuring step (S108-S114) to measure second voltage values at each of the plurality of current measuring parts in a state in which a current flows in one of the plurality of socket parts; a correction formula calculating step (S116) to calculate a correction formula formed by a inverse matrix of a matrix having as its elements differences between the second voltage values and the first voltage values, characterized in that there are provided: a third measuring step (S604-S610) to measure third voltage values output from each of a plurality of current measuring parts in a state in which one or more electrical apparatuses are electrically connected to the power strip; and an initial value calculating step (S118) to calculate initial values from differences between the third voltage values and the first voltage values, wherein a third measuring step (S604-S610) to measure third voltage values output from each of a plurality of current measuring parts in a state in which one or more electrical apparatuses are electrically connected to the power strip; and an initial value calculating step (S118) to calculate initial values from differences between the third voltage values and the first voltage values, wherein the first measuring step, the second measuring step, the third measuring step, and the correction formula calculating step are performed for each of a plurality of different temperatures of the power strip, and wherein the initial values and the correction formula are used to calculate currents flowing in the plurality of socket parts of the power strip. <EOS>