To provide an isothrtmal agitation apparatus in which the service life of a magnetic stirrer is extended while controlling the temperature of a solution in a vessel with high precision and the shape of a liquid tank is devised to prevent the vessel to be put in the liquid tank from floating up.

This isothrtmal agitation apparatus is provided with: the isothrtmal liquid tank 12 having a bottom wall 16 which is formed to have a difference in level, namely, have a shallow portion 16b and a deep portion 16c and at least the shallow portion 16a of which is made from a magnetically permeable material; a means 14 arranged in the deep portion 16c of the isothrtmal liquid tank 12 for controlling the temperature of the water W stored in the isothrtmal liquid tank 12; and the magnetic stirrer 13 which is arranged in the external space below the bottom wall of the shallow portion 16b of the isothrtmal liquid tank 12 and is used for generating a magnetic field to rotate a magnetic agitating bar 5 which is put in a beaker 4 to be placed on the bottom wall of the shallow portion 16b of the isothrtmal liquid tank 12 together with the solution Y.