An apparatus for high-temperature measurement of magnetic susceptibility using the modified Gouy method was developed. It can be used at temperatures of up to 1200 °C. The apparatus consists of the heating system, weighing system, and temperature-programming controller. The sample should be placed at a position of relatively constant B(dB/dz) in a gradient magnetic field. Each crucible is used only once for a single measurement to reduce the error. The reliable accuracy and good repeatability of the apparatus are demonstrated by measuring the magnetic susceptibilities of a DD483 superalloy, Ni-based solid solution alloy, and Ni3Al-based solid solution alloy at various temperatures. The results also successfully explain the change in the γ' precipitation in DD483 superalloys. Therefore, the apparatus provides a convenient and powerful tool for investigating high-temperature phase transitions in the presence of a magnetic field.