Electrochemical reaction device and electrochemical reaction method

An electrochemical reaction device, comprising:

• an electrochemical reaction cell that includes a first electrode having a first flow path, a second electrode having a second flow path, and a separating membrane sandwiched between the first electrode and the second electrode and having a polymer electrolyte membrane;
• a liquid to be treated supply system that includes a pure-water production unit to produce pure water and an ultrapure-water production unit to produce ultrapure water by treating the pure water supplied from the pure-water production unit, and supplies the ultrapure water to the electrochemical reaction cell as a liquid to be treated;
• and
• a liquid tank configured to receive the ultrapure water to be supplied from the electrochemical reaction cell and to be treated in the electrochemical reaction cell and the pure water to be supplied from the pure-water production unit, wherein
• the liquid to be treated supply system includes a pure water and treated ultrapure water backflow suppression mechanism that suppresses backflow of the pure water and the treated ultrapure water in the liquid tank, from the liquid tank into the electrochemical reaction cell.