The present invention facilitates the construction of a waterproof sensor assembly having, for example, a sensor main body arranged in a vehicle seat, a sensor terminal, a sensor terminal protection portion, a first packing having a hollow ring shape, a second packing having a hollow ring shape, a connector casing having a hollow ring shaped opening, and a connector cover for covering the opening connector casing opening. The sensor terminal protection portion has a hollow ring shape and is integrated with a periphery of the sensor main body. The sensor terminal is integrated with the sensor terminal protection portion such that the sensor terminal protrudes from an inner periphery of the sensor terminal protection portion toward an inside of the sensor terminal protection portion. The arrangement of the first and second packing and the connector case and cover provide a high performance waterproof sealing of the sensor and parts thereof.