To achieve an environmentally sensitive switch that is used for turning on/off power supply to an electric circuit of an alarm sounder or the like while detecting an external environment with a simple and inexpensive configuration.

The environmentally sensitive switch is provided with a casing which has a first planar electrode on an inner-wall face and is deformed corresponding to an external environment in contact with an outer-wall face, an electrode structure composed by radially installing a plurality of second needle-like or subulate electrodes, and an insulating support that supports the electrode structure in the casing so as to prevent the first/second electrodes from being in contact with each other. The insulating support is elastic and formed into a pin-like shape or a sponge-like shape. The first/second electrodes are brought into contact with each other corresponding to the deformation of the casing. Consequently, the environmentally sensitive switch functions as a switch element.