  The emission-line flux ratio of [O III]4363/[O III]5007 is a useful
diagnostic for the ionization mechanism and physical properties of narrow-line
regions in active galactic nuclei. However, it is known that simple
photoionization models underpredict this ratio. In this contribution, we report
on some pieces of evidence that a large fraction of the [O III]4363 emission
arises from the dense gas, which can be obscured by dusty tori. Taking this
dense component into account, we show that the flux ratio of [O III]4363/[O
III]5007 can be explained by two-component photoionization models.
