A solution of ammonium thiocyanate is used in the FIP (Fédération International de la Précontrainte) test as a hydrogen charging method. Though this method is comparatively simple, fracture time in the FIP test and hydrogen content often differ among various testing institutes. However, the detailed hydrogen absorption behavior in the solution is still not clear. In this context, the effects of existing states of hydrogen, oxide film on the specimen surface, specific solution volume to specimen surface area, immersion time and solution temperature on the hydrogen absorption behavior of a steel bar for reinforcing prestressed concrete were investigated by immersing it in the solution. The amount of absorbed hydrogen increased with immersion time, reached its maximum, and then decreased with increasing immersion time. A main factor of the decrease in the amount of absorbed hydrogen was corrosion products, including Fe, O and S, formed on the specimen surface, since the amount of absorbed hydrogen increased again as a result of merely polishing the surface. This indicates that corrosion products formed on immersing specimens in a solution of ammonium thiocyanate strongly affect hydrogen absorption behavior. Whereas, variation of the solution, such as increase in pH, during immersion also affects slightly hydrogen absorption behavior.