A demodulation unit for a GNSS signal processing device includes an operator that selects an error detecting method based on signs of early and late differential values and calculates an error detection value. A code phase range where an error detection value is not 0 is wide with a first error detecting method, and is narrow with a second. Immediately after capturing a GNSS signal, a code phase difference between the GNSS signal and a prompt replica signal is large, and signs of the early differential value and the late differential value are different from each other. In this case, the first method is used. As the code phase is driven, the code phase difference between the GNSS signal and the prompt replica signal becomes smaller, and the signs of the early differential value and the late differential value become the same. In this case, the second method is used.