To provide an inclination detector and the like for detecting feeble inclination or vibration in a simple configuration.

In the inclination detector, liquid and gas mutually come into contact so as to form an interface. Light-emitting diode (LED) and phototransistor (PT) are positioned so that light emitted from the LED enters the gas, penetrates through the interface to enter the liquid, and then is received by the phototransistor PT. The PT receives the light emitted from the LED to generate signal indicating light intensity of its received light. A variation in this signal is negatively fed back to change in brightness of the LED, the amount of light received by the PT converges on the prescribed amount. Transistor Q1 generates signal indicating a signal size of current flowing through the LED as the signal indicating amount of tilt in interface.