An electric construction machine comprising: an electric motor; a hydraulic pump driven by the electric motor; hydraulic actuators (14, 18, 19, 20), each driven by a hydraulic fluid delivered from the hydraulic pump; an internal battery that is an electric power supply source for the electric motor; a plurality of connectors (59A, 59B, 59C) to which respective plugs (60A, 60B, 60C) for a plurality of kinds of external power supplies (49A, 49B, 49C) are adapted to be connected; a plurality of power transmission systems (71A, 71B, 71C) each connected to one of the plurality of connectors (59A, 59B, 59C); characterized by a battery charger to which the plurality of power transmission systems (71A, 71B, 71C) are connected via one common system, the battery charger being configured so that when any one of the plurality of connectors (59A, 59B, 59C) is connected to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C), the battery charger supplies, to the internal battery, electric power supplied from the corresponding external power supply, wherein the electric construction machine further comprises: a plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), each disposed in one of the plurality of power transmission systems (71A, 71B, 71C) and each associated with one of the plurality of connectors (59A, 59B, 59C); a detection block that detects whether one of the plurality of connectors (59A, 59B, 59C) has been connected to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C) or whether one of the connectors (59A, 59B, 59C) has been selected for connection to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C); and a switching control block that conducts switching control so that among the plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), only the switches associated with the connector detected by the detection block will close and all the other switches will open. an electric motor; a hydraulic pump driven by the electric motor; hydraulic actuators (14, 18, 19, 20), each driven by a hydraulic fluid delivered from the hydraulic pump; an internal battery that is an electric power supply source for the electric motor; a plurality of connectors (59A, 59B, 59C) to which respective plugs (60A, 60B, 60C) for a plurality of kinds of external power supplies (49A, 49B, 49C) are adapted to be connected; a plurality of power transmission systems (71A, 71B, 71C) each connected to one of the plurality of connectors (59A, 59B, 59C); characterized by a battery charger to which the plurality of power transmission systems (71A, 71B, 71C) are connected via one common system, the battery charger being configured so that when any one of the plurality of connectors (59A, 59B, 59C) is connected to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C), the battery charger supplies, to the internal battery, electric power supplied from the corresponding external power supply, wherein the electric construction machine further comprises: a plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), each disposed in one of the plurality of power transmission systems (71A, 71B, 71C) and each associated with one of the plurality of connectors (59A, 59B, 59C); a detection block that detects whether one of the plurality of connectors (59A, 59B, 59C) has been connected to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C) or whether one of the connectors (59A, 59B, 59C) has been selected for connection to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C); and a switching control block that conducts switching control so that among the plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), only the switches associated with the connector detected by the detection block will close and all the other switches will open. a plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), each disposed in one of the plurality of power transmission systems (71A, 71B, 71C) and each associated with one of the plurality of connectors (59A, 59B, 59C); a detection block that detects whether one of the plurality of connectors (59A, 59B, 59C) has been connected to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C) or whether one of the connectors (59A, 59B, 59C) has been selected for connection to corresponding one of the plugs (60A, 60B, 60C) for the external power supplies (49A, 49B, 49C); and a switching control block that conducts switching control so that among the plurality of switches (74Aa, 74Ab; 74Ba, 74Bb; 74Ca, 74Cb), only the switches associated with the connector detected by the detection block will close and all the other switches will open.