A signal processing device and a measuring method are provided. A ring oscillator includes (2n+1) signal transmission circuits (n is an integer greater than or equal to 1). One of the signal transmission circuits comprises an inverter, a first transistor, and a second transistor; one of an input terminal and an output terminal of the inverter is connected to one of a source and a drain of the first transistor; one of a source and a drain of the second transistor is connected to a gate of the first transistor; an output of a k-th (k is an integer greater than or equal to 1 and less than or equal to 2n) signal transmission circuit is connected to an input of a (k+1)-th signal transmission circuit; and an output of a (2n+1)-th signal transmission circuit is connected to an input of a first signal transmission circuit.