There is provided an in-furnace temperature measuring method that is capable of reducing the number of operation steps that are required for temperature measurement, and effectively applying a measurement result even if colors and finishing states of a circuit board and an electronic part are changed. First and second pseudo circuit boards (12) and (13) having the substantially same configuration and dimensions as those of a circuit board are inserted into a reflow furnace (11), the front and rear surface temperatures of the first and second pseudo circuit boards (12) and (13) and air temperatures around the first and second pseudo circuit boards (12) and (13) within the reflow furnace (11) are measured. The entire surface of a metal whose physical value is known is black-coated in the first pseudo circuit board (12), and the entire surface of a metal whose given physical value is known is mirror-finished in the second pseudo circuit board (13).