A small, compact negative‐ion source based on plasma sputtering has been developed for secondary‐ion mass spectrometry (SIMS). It can make precise measurements of the oxygen isotope ratios in minerals. From a copper sputter target, a high‐density63Cu−ion beam of 3 mA cm−2has been obtained as the primary‐ion beam with a size of 100 &mgr;m in diameter. The mass distribution of the ion beam is 99% Cu−and 1% Cu−2ions; other ionic species comprise less than 0.1%. A typical beam stability for 10 min has been achieved at 0.4% (&sgr;). These performances of the negative‐ion source are useful to performinsituanalyses of insulator materials in SIMS. As an application, the high‐mass resolution spectra for three isotopes of oxygen secondary ions from an insulator sample bombarded by the63Cu−ion beam was studied.