Phosphonium borate compounds of formula (1) are novel. R1, R2, R3 and R4 each are a monovalent organic group having an aromatic or heterocyclic ring or a monovalent aliphatic group, at least one of R1, R2, R3 and R4 which are bonded to phosphorus atom and at least one of R1, R2, R3 and R4 which are bonded to boron atom are each an organic group derived by releasing a proton from a proton donor selected from among an aromatic carboxylic acid having at least one carboxyl group, an aromatic carboxylic acid having at least one acid anhydride group and at least one carboxyl group, a phenol compound having at least one phenolic hydroxyl group, an aromatic compound having at least one carboxyl group and at least one phenolic hydroxyl group, and an aliphatic carboxylic acid having at least one carboxyl group. The phosphonium borate compounds are useful as a latent curing catalyst for epoxy resin compositions. The epoxy resin compositions are improved in shelf stability, fast-curing ability and flow and cure into products having moisture resistance and improved electrical properties.