PURPOSE: To provide a device and a method for measuring the temperature of a reactive metal workpiece sealed in a retort before and during diffusion joining of a workpiece.

CONSTITUTION: This device comprises temperature measuring devices 210, 212, 214, 220, 222, 224 adjacent to prescribed parts of a workpiece in a retort 100, a thermal conducting means for connecting these devices to a display device outside the retort 100, conduits 230, 240 including first and second terminal parts, which are connected to the interior of the retort 100 and the display device, and a means for sealing the first and second terminal parts for keeping the exhaust state in the retort 100 after evacuated. This method comprises the steps of measuring the temperature of a prescribed part of a workpiece in the retort 100 in the course of a diffusion joining process and controlling the temperature given to the retort 100 in the course of the subsequent diffusion joining process using the measured temperature.