This passive one-way valve is used in a connection portion between first and second flow channels, and includes: an inlet portion into which a fluid from the first flow channel flows; an elastic portion for blocking the inlet portion; a rigid portion for supporting the elastic portion from a side opposite to the inlet portion across the elastic portion; a gap portion formed around the rigid portion; and an outlet portion which is in communication with the gap portion for letting the fluid flow out into the second flow channel. Furthermore, the inlet portion is hermetically sealed by the elastic portion being pressed against the inlet portion side by the rigid portion. According to this passive one-way valve, it is possible to provide a simple and inexpensive passive one-way valve without requiring special equipment such as a vacuum pump and pressurized air, and to provide a microfluidic device using the same.