PURPOSE: To readily obtain the titled compound useful as a dye intermediate in high selectivity of catalyst, by using a catalyst containing a specific active component compound in a given ratio and bringing a gas mixture comprising acenaphthene and a molecular oxygen-containing gas into contact with the catalyst.

CONSTITUTION: A catalyst which is obtained by supporting 20W200g, preferably 40W150g based on 1l nonporous inert carrier of a catalytic component comprising 70W90wt.%, preferably 75W85wt.% titanium dioxide, 10W30wt.%, preferably 12W22wt.% vanadium pentoxide and 0.5W5wt.%, preferably 3W4.5wt.% (calculated as sulfate) alkali metallic compound on the nonporous carrier and substantially contains neither phosphorus compound nor niobium compound in the catalytic active component is used as the catalyst and a gas mixture comprising acenaphthene and a molecular oxygen-containing gas is brought into contact with a catalytic layer packed with the catalyst and oxidized to give the aimed compound.