A simple device to measure electrical resistivity has been developed for field and laboratory use. The measurement system comprises a probe unit, current wave generator, amplifier, A/D converter, data acquisition unit with RS-232C interface and notebook personal computer. The system is applicable to soils and soft rocks as long as the probe needles can pierce into them. Frequency range of the measurement system extends from 100 Hz to 10 MHz. The total error of the system is less than 5 %. in situ measurements of the resistivity and shear resistance by means of pocket-sized penetro-meter were applied to Pleistocene clayey beds. Some laboratory tests were also conducted to examine the interpretation of the in situ resistivity. Marine and non-marine clayey sediments are different in their resistivities of the stratum by in situ test and the clay suspension sampled from the strata. Physical and mechanical properties were compared with the resistivity and general relationships among them were explored to clarify the characteristics of inter-particle bonding. Some possible mechanism regarding the peculiar weathering of clayey sediment or mudstone beds is discussed from the viewpoint of physico-chemical process, which is conspicuous especially near the ground surface.