A hydrogen manufacturing system for performing offgas flow control includes: a vaporizer (1) for heating a material mixture containing a hydrocarbon material; a reforming reactor (2) for generating hydrogen-containing reformed gas by reforming reactions of the material; a PSA separator (5) for repeating a cycle of adsorption and desorption, where in the adsorption PSA separation is performed with an adsorption tower loaded with an adsorbent to adsorb unnecessary components in the reformed gas and extract hydrogen-enriched gas out of the tower, and in the desorption the offgas containing the unnecessary components from the adsorbent and remaining hydrogen is discharged from the tower; and a buffer tank (6) for holding the offgas before supplying to the vaporizer. The offgas flow supply from the tank (6) to the vaporizer is changed continuously over time when the cycle time is changed according to load change on the separator (5).