To provide an electric power supply device for a machine for tunneling capable of easily extending a tough-rubber sheath cable or housing it in accordance with the movement of a machine for tunneling when connecting the machine and a relay transformer through the rubber sheath cable to supplying power.

This electric power supply device is constituted of at least one power relay transformer 12 disposed in an excavation tunnel and a crawler dump truck 13 in which a current-collecting reel 13b is mounted on a staging to freely wind or wind off the tough-rubber sheath cable to the current-collecting reel 13b. The power relay transformer 12 and a transferable machine for tunneling are electrically connected through the rubber sheath cable 15 wound on the current collecting reel 13b installed in the crawler dump truck 13 to supply electric power to the machine for tunneling and transfer the crawler dump truck 13 in accordance with the movement of the machine or let off the tough- rubber sheath cable 15 out of the current collecting reel 13b or wind it thereon.