A valve device with fail-safe mechanism having: a valve element housing having a valve element side opening; a communication port provided in the valve element housing; a fail-safe mechanism; and a valve element rotatably housed in the valve element housing, switching communication between the valve element side opening and the communication port; the fail-safe mechanism including: a thermo-element; and an element housing that accommodates the thermo-element, the element housing including: a small-diameter housing portion that houses a thermo-element; a large-diameter housing portion having an inner diameter larger diameter than an inner diameter of the small-diameter housing; and a step formed between the small-diameter housing portion and the large-diameter housing portion, one of the small-diameter housing portion and the large-diameter housing portion is provided with a valve element communicating portion communicating with the valve element housing, the fail-safe mechanism including: a closing member that closes the step; and a biasing portion that biases the closing member toward the thermo-element, an adapter provided to the communication port, the adapter having a through-hole for communicating the other of the small-diameter housing portion and the large-diameter housing portion with the communication port, characterized in that a seal retention member is disposed in the communication port, the adapter is made of plastic and includes an enlarged diameter portion to hold down the seal retention member and a prevention hole for preventing depressions due to molding shrinkage occurring in the enlarged diameter portion, and the through-hole is formed so as to penetrate the prevention hole a valve element housing having a valve element side opening; a communication port provided in the valve element housing; a fail-safe mechanism; and a valve element rotatably housed in the valve element housing, switching communication between the valve element side opening and the communication port; the fail-safe mechanism including: a thermo-element; and an element housing that accommodates the thermo-element, the element housing including: a small-diameter housing portion that houses a thermo-element; a large-diameter housing portion having an inner diameter larger diameter than an inner diameter of the small-diameter housing; and a step formed between the small-diameter housing portion and the large-diameter housing portion, a thermo-element; and an element housing that accommodates the thermo-element, the element housing including: a small-diameter housing portion that houses a thermo-element; a large-diameter housing portion having an inner diameter larger diameter than an inner diameter of the small-diameter housing; and a step formed between the small-diameter housing portion and the large-diameter housing portion, a small-diameter housing portion that houses a thermo-element; a large-diameter housing portion having an inner diameter larger diameter than an inner diameter of the small-diameter housing; and a step formed between the small-diameter housing portion and the large-diameter housing portion, one of the small-diameter housing portion and the large-diameter housing portion is provided with a valve element communicating portion communicating with the valve element housing, the fail-safe mechanism including: a closing member that closes the step; and a biasing portion that biases the closing member toward the thermo-element, a closing member that closes the step; and a biasing portion that biases the closing member toward the thermo-element, an adapter provided to the communication port, the adapter having a through-hole for communicating the other of the small-diameter housing portion and the large-diameter housing portion with the communication port, characterized in that a seal retention member is disposed in the communication port, the adapter is made of plastic and includes an enlarged diameter portion to hold down the seal retention member and a prevention hole for preventing depressions due to molding shrinkage occurring in the enlarged diameter portion, and the through-hole is formed so as to penetrate the prevention hole