A valve timing control device of internal combustion engine comprises a chain case 6 that is fixed to a cylinder head 101 of the engine and has a circular opening 55 for receiving therein a cylindrical housing 5a of an electric motor 8, an annular seal member 58 that is operatively received in an annular clearance defined between an outer cylindrical wall of the cylindrical housing 5a and an inner cylindrical wall of the circular opening 55, and a cover member 4 that is connected to the chain case 6 to cover the circular opening thereby concealing the annular seal member from the outside, wherein when the cover member 4 is removed from the chain case 6, the annular seal member 58 becomes exposed to the outside through the circular opening 55 of the chain case 6 for a visual inspection of the annular seal member.