A fixed-point cell for connection with a thermometer protecting tube and apparatus for estimating the lifetime of thermometerin the thermometer protecting tube including: a reference temperature material layer made of high-purity metallic materials to indicate a reference temperature based on a phase change of material, which coexists in a two-phase or three-phase state; a container part of a multiple structure having the reference temperature material layer embedded and sealed therein for achieving thermal correlation between the reference temperature material layer and the thermometer protecting tube having a thermometer contained therein, the thermometer protecting tube being inserted into the central hole of the fixed-point cell while being in contact with the inner circumference of the container part; and fixing devices disposed on the top portion of the container part for fixing the thermometer protecting tube to the container part in a fastening manner.