The polydimethylsiloxanes in the defoamants forming part of the lubricant compositions of the present invention are represented by the following formula (I) . [Chem.l] C H a C H 3 C H , I I I C H ., - S i - O -(~ S i - 0 -) - S i - C H 8 ??? ??? ??? ( I ) I I . 1 C H J C H a C H n (in the aforesaid formula I, n is a positive integer and is a value corresponding to the viscosity) .