There are provided an operating mechanism that drives the opening/closing of contacts, and a sliding conduction part that forms electrical connection of these contacts and the main circuit on receipt of a connection voltage; grease is applied to the mechanical sliding part of the operating mechanism and to the sliding conduction part. The grease has as base oil at least one synthetic oil and is selected from the group consisting of poly &agr;-olefins and dialkyl diphenyl ethers, and contains as thickener 5-30 mass % of a urea compound. The dynamic viscosity of the base oil is 30-500 mm2/s at 40° C.