A charge equalization apparatus for a series-connected battery string, comprising: a battery string consisting of a plurality of battery cells connected in series; a nth battery cell (Bn) belonging to said battery string; a battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) which is one portion of the battery string and has m battery cells connected in series and including the nth battery cell (Bn), said apparatus further comprising an automatic PWM generating means (310; 610; 710; 810; 910; 1210; 1310; 1410; 1510) connected in parallel to each of the battery cells composing the battery string; and a charge equalizer (320; 620; 720; 820; 920; 1220; 1320; 1420; 1520) comprising a DC/DC converter connected in parallel to each of the battery cells composing the battery string, wherein the automatic PWM generating means (310; 610; 710; 810; 910; 1210; 1310; 1410; 1510) comprises a comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512) and a PWM generator (311; 611; 711; 811; 911; 1211; 1311; 1411; 1511) automatically generating a PWM signal by an output of the comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512); said comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512) generating an output by comparing a potential of the nth battery cell (Bn) with an average potential of the whole battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) including the n th battery cell, and said charge equalizer (320; 620; 720; 820; 920; 1220; 1320; 1420; 1520) automatically controlling the charge or discharge of the nth battery cell (Bn) according to the PWM signal automatically generated by the PWM generator (311; 611; 711; 811; 911; 1211; 1311; 1411; 1511), and wherein an input or an output of the DC/DC converter is connected to the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040), so that charging current of the nth battery cell (Bn) is caused by total potential of the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) when charging the nth battery cell (Bn) and discharging current of the nth battery cell (Bn) serves as total potential of the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) when discharging the nth battery cell (Bn), wherein, the nth battery cell (Bn) refers to any single battery of the plurality of series-connected batteries composing the battery string, and m is at least 3 and up to the number (k) of total batteries composing the battery string. a battery string consisting of a plurality of battery cells connected in series; a nth battery cell (Bn) belonging to said battery string; a battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) which is one portion of the battery string and has m battery cells connected in series and including the nth battery cell (Bn), said apparatus further comprising an automatic PWM generating means (310; 610; 710; 810; 910; 1210; 1310; 1410; 1510) connected in parallel to each of the battery cells composing the battery string; and a charge equalizer (320; 620; 720; 820; 920; 1220; 1320; 1420; 1520) comprising a DC/DC converter connected in parallel to each of the battery cells composing the battery string, wherein the automatic PWM generating means (310; 610; 710; 810; 910; 1210; 1310; 1410; 1510) comprises a comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512) and a PWM generator (311; 611; 711; 811; 911; 1211; 1311; 1411; 1511) automatically generating a PWM signal by an output of the comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512); said comparator (312; 612; 712; 812; 912; 1212; 1312; 1412; 1512) generating an output by comparing a potential of the nth battery cell (Bn) with an average potential of the whole battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) including the n th battery cell, and said charge equalizer (320; 620; 720; 820; 920; 1220; 1320; 1420; 1520) automatically controlling the charge or discharge of the nth battery cell (Bn) according to the PWM signal automatically generated by the PWM generator (311; 611; 711; 811; 911; 1211; 1311; 1411; 1511), and wherein an input or an output of the DC/DC converter is connected to the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040), so that charging current of the nth battery cell (Bn) is caused by total potential of the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) when charging the nth battery cell (Bn) and discharging current of the nth battery cell (Bn) serves as total potential of the battery string of m battery cells (330; 410, 420, 430, 440; 1010, 1020, 1030, 1040) when discharging the nth battery cell (Bn), wherein, the nth battery cell (Bn) refers to any single battery of the plurality of series-connected batteries composing the battery string, and m is at least 3 and up to the number (k) of total batteries composing the battery string.