It was attempted to grow microneedles of tungsten on 10-μm-diam tungsten wires for the purpose of preparing emitters of high efficiency for field-ionization mass spectrometry. A large quantity of tungsten needles with dendritic structures was successfully grown by reducing vapor-deposited tungsten oxide with 1–5×10−2 Torr hydrogen while applying a positive potential of 5–6 kV. The needle growth occurred in a critical manner when deposited tungsten oxide attained a temperature of ∠1200 K, and needles obtained were thin enough to easily generate high electric fields of 107–108 V/cm. Testing in a mass spectrometer actually verified that the needles can yield field-ionization mass spectra of satisfactory quality for typical organic substances.