A new type of ocean bottom magnetometer (OBM) for investigations of conductivity anomaly (CA) was built according to the philosophy of design that it should be small in size, light in weight, easy to operate, strong enough against mechanical shocks, capable of long-term observations with an accuracy better than 1nT. The device of the pop-up type OBM has several characteristics as follows; small fluxgate sensors are used for magnetic detection, outputs of sensors are converted to digital data and stored in magnetic bubble memories, electric power is supplied intermittently for economy's sake; the measurement system is installed in a pressure-tight glass sphere with an outer diameter of about 43cm, and its recovery from the ocean bottom is made by sending an ultrasonic command from a ship to the releasing system of the OBM.After a successful test on land, trial observations were carried out in the Sagami Trough at the water depth of about 1, 500m. Records were obtained for the whole period of observation from November 13 to December 16, 1982, and analyzed to give transfer functions and Parkinson vectors. The results seem to be consistent with the central Japan anomaly for geomagnetic variation with a period longer than ten odd minutes. For the period of ten minutes, other type of conductivity anomaly seems to exist.