Beta-ray thickness gage requires a pre-amplifier of high input impedance for amplification of small input current of 10-11∼10-12 amp. generated in the ionization chamber. A solid-state circuit, which is considered ideal to be used in automation instruments, is realized by using the technique of parametric oscillating amplifier incorporating a “Pico-diode”-a variable capacitance diode with very low leakage current. This amplifier is entirely reliable and perfect for industrial instruments. The β-ray thickness gage with the solid-state electrometer (pico-diode electrometer) requires no warm-up time. Since it needs only small amount of radio isotope for its high sensitivity and stability, safety is improved, too. Given in this paper are features of this β-ray thickness gage, principle and characteristics of the solid-state electrometer, analysis of transfer characteristics of the oscillating amplifier and some of their applications.