A dynamic quantity detection device includes a detection portion, a signal process portion, and an operation mode switch portion. The detection portion detects a dynamic quantity applied from an outside and generates a sensor signal in accordance with the dynamic quantity. The signal process portion processes the sensor signal from the detection portion to have a property suitable for an external device. When the sensor signal output from the detection portion is less than or equal to a predetermined threshold value, the operation mode switch portion sets an operation mode to a power save mode by stopping an operation of the signal process portion. When the sensor signal output from the detection portion is greater than the predetermined threshold value, the operation mode switch portion switches the operation mode to a normal mode by activating the signal process portion.