A hydrogen generating device is adapted for a fuel cell. The hydrogen generating device includes a casing, a button, a solid reactant, a bag-shaped body, and at least one flexible element. The casing has a containing space and an opening. The button is integrally formed and connected to the casing to seal the opening. The solid reactant is disposed in the casing. The bag-shaped body is disposed in the casing and contains a liquid reactant. The flexible element is connected to the casing and is located in the containing space. The flexible element includes a bending end, wherein the flexible element is aligned to the button and is located between the button and the bag-shaped body. When the button is pressed, the button pushes the flexible element so the bending end pierces the bag-shaped body, and the liquid reactant flows out and reacts with the solid reactant to generate hydrogen.