A phosphonium borate compound represented by the general formula (I) (compound (I)). Provided are: A) a new process in which the compound (I) is safely produced on an industrial scale in a high yield through simple reaction operations; B) the novel compound, which is easy to handle; and C) a novel use in a catalyst. General formula (I): (R<1>)(R<2>)(R<3>)PH.BAr4 [In the formula, R<1>, R<2>, R<3>, and Ar are the same as in the description.] A) The process is characterized by reacting a phosphine with a) HCl or b) H2SO4 to yield a) a hydrochloric acid salt or b) a sulfuric acid salt and then reacting the salt with a tetraaryl borate compound. B) The compound (I) is characterized in that it has a secondary or tertiary alkyl or the like as R<1> and can be easily handled even in air without necessitating special caution. C) The use is in a transition metal complex catalyst for use in reactions for C-C, C-N, and C-O bond formation, and is characterized in that the compound (I) is used in place of an unstable phosphine compound to produce the same effect.