A phased-array antenna device with a long operating life without mechanical parts, has a high spatial resolution, and realizes microwave observation of broadband and high-frequency resolution. The phased-array antenna device performs direct A/D conversion through a BPF on an antenna analog signal amplified by an amplifier. Then, the device performs a second cross-spectrum calculation after conversion into complex frequency data through FFT. To detect a weak electromagnetic wave, the device repeatedly performs a second FFT over a long period and lastly performs integration.