gives excellent antifungal activity in comparison with oit or bbit solely further it is extremely less corrosive to metal than cmit and shows long residual activity than cmit so the composition is easy to be handled and is very economical the present invention also provides a wood preservative which comprises oit bbit copper oxide or a copper salt and a fungicidal azole as active ingredients the wood preservative is usually prepared by adding a mixture of oit and bbit or formulation thereof to a wood preservative containing copper oxide or a copper salt and a fungicidal azole the wood preservative is usually aqueous solution emulsion or suspension and the weight ratio of oit and bbit in the wood preservative is usually 1 19 to 19 1 preferably 1 9 to 9 1 each content of oit bbit copper oxide or a copper salt and a fungicidal azole is usually 0 01 30 0 01 30 50 or more and 0 01 30 by weight respectively the wood preservative is generally applied into timber at the volume of 200 to 500 kg m 3 by injection under high pressure the following examples and test examples of the present invention are set forth by way of illustration but the present.