An apparatus for producing hydrogen is able to measure a position of a interface or a surface boundary of liquids in a reaction vessel without direct contact. An apparatus for producing hydrogen by an IS process includes a reaction vessel, a radiation source, a radiation detector, and a data processing unit. Reacting liquids are to be introduced in the reaction vessel. The radiation source is provided at a first side wall of the reaction vessel. The radiation detector is provided at a second side wall which faces the first side wall provided with the radiation source. The data processing unit is connected to the radiation detector, which receives radiation data transmitted through the reaction vessel, from the radiation detector. The radiation data processing unit estimates constituents and concentrations of the reacting liquids from the radiation data.