(Accounting Standard for Financial Instruments 20-21)
For securities, where the fair value decreases dramatically (unless there is a possibility of recovery), the carrying value of those financial assets is reduced to fair value (market value), and the related loss is recognised in profit or loss.
For shares whose market value is extremely difficult to measure, if the value in substance decreases dramatically, the carrying amount shall be reduced to the in substance value and the loss shall be accounted for through profit or loss.
(Practical guidance on Accounting Standard for Financial Instruments 93)
It is assumed that the market value of a security is extremely difficult to measure.