Freeze-drying device and freeze-drying method

A freeze-drying method includes depressurizing containers filled with a liquid including a raw material and a medium with a freeze-drying device to freeze the liquid from a liquid surface. The depressurizing includes executing an exhaust mitigation process that performs the depressurizing at an exhaust capability that is less than a rated exhaust capability of the freeze-drying device, and using a partial pressure value of the medium to determine when the exhaust mitigation process ends. The executing an exhaust mitigation process includes maintaining an exhaust speed of a gas capture pump configured to discharge gas from a freeze-drying chamber accommodating the containers, and decreasing an exhaust speed of a positive-displacement pump configured to discharge gas from a space accommodating the gas capture pump.