To provide a power supply for a built-in type tire pressure sensor capable of directly putting/taking a cell on/off at outside of a vehicular tire by providing the tire pressure sensor in the vehicular tire and mounting the cell on an outer side of the tire.

This power supply is provided with a nozzle casing 7 and a power source case 6 to be screwed to an outer end part 71 of the nozzle casing 7 in an outer part of a tire 3, and a cell is mounted to the power source case 6 followed by electric connection of one electrode of the cell to the nozzle casing 7 through the power source case 6 while the other electrode electrically connects to an tire pressure sensor 4 built in the tire 3 through a valve core electrically insulatingly inserted in the nozzle casing 7 to supply power from the cell in the outer part of the tire 3 to the built-in type tire pressure sensor 4.