An improved heat-insulating support device is disclosed which is capable of supporting a coolant tank to a vacuum vessel in a manner such that when the vacuum tank is subjected to a great external force after the coolant tank has been cooled down to a cryogenic temperature, the coolant tank and hence cryogenic equipment mounted thereon are held in place and prevented from being displaced beyond a prescribed allowable range, without impairing the intended heat-insulating capability thereof. The heat-insulating support device comprises a heat insulating support member disposed outside the coolant tank with one end connected with the coolant tank; a mounting rod having one end connected with the heat-insulating support member and its other end projected outwardly through the vacuum vessel; and a spring for resiliently mounting the other projected end portion of the mounting rod on the vacuum vessel, the spring having a non-linear spring characteristic including a small spring constant for mainly absorbing thermal contraction of the coolant tank and a large spring constant for mainly suppressing displacement of the coolant tank due to large external forces acting thereon after the thermal contraction of the coolant tank.