An in-vehicle fuel cell stack includes a ventilation mechanism for ventilating a stack case through a ventilation opening opened to the inside of the stack case. The ventilation mechanism includes an inlet member. An air inlet port and an inner channel are formed in the inlet member. A ventilating air is guided into the ventilation mechanism through the air inlet port. The inner channel connects the air inlet port and the ventilation opening. The inner channel includes a labyrinth channel.