To provide a security device capable of obtaining a high crime-prevention function while reducing power consumption.

This security device 100 comprises a sensor 101 for detecting a suspicious person, light control glass 102 capable of changing light transmittance, a light control glass control unit 103 for controlling light transmittance of the light control glass 102, and a periphery detection device 104 for instructing the change of light transmittance to the light control glass control unit 103.