To provide a base isolating device capable of switching a lock valve without employing a solenoid valve.

A base isolating device includes a lock valve 10 which opens and closes a passage 9 between a cylinder 1 and a damping valve 11 for damping a stretching energy of a cylinder 1 expanding and contracting in response to a tremor and with a switching valve 16 which cuts off the communication between a pilot chamber of the lock valve 10 and tank T to force the lock valve 10 to exert a locking function at ordinary time and communicates the pilot chamber to the tank T to release the locking function of the locking valve 10, and damps the stretching energy of the cylinder 1 by the damping valve 11 when the locking function of the lock valve 10 is released, wherein the switching valve 16 contains a pilot chamber 16c for introducing a pilot pressure supplied on the basis of a signal when earthquake occurs. When switched to a releasing position 16a for releasing the locking function of the lock valve 10 by pressure action of the pilot chamber 16c, the pilot chamber 16c is communicated with the tank T.