Distribution of inorganic compounds in the forested catchment was estimated by the relationship between catchment area and concentration of inorganic compounds content in flood runoff water. Water quality about K^+, NO_3^--N, SiO_2 (=inorganic compounds dissolved in interflow) were measured at three points along same stream. They located at the outlets of catchments with different area as each fits the others. The temporal changes and the averages of concentration were analyzed during three rainfall periods. The maximum concentration about K^+ and NO_3^--N were showed before and at 3 or 4 hour after the peak runoff respectively. Temporal changes of concentration about SiO_2 tend to be contrary to the changes of runoff. The average concentration about NO_3^--N showed large value and that about SiO_2 showed small value as increment of catchment area. These analyses led that the catchment area was efficiency to the concentration of inorganic compounds in runoff water according to the roots of interflow in soil layers.