A low noise ion detector for microimpurity analysis with a quadrupole mass spectrometer has been developed using the off‐axis technique with an electrostatic deflector plate and a channel electron multiplier. This arrangement of ion detector can greatly reduce the noise and baseline shifts caused by photons and excited neutrals, which are dependent on the emission current and pressure in the ion source. The ion collection efficiency of 100% in this arrangement is obtained at a multiplier voltage of −3 kV and a deflector voltage of 10 V. The noise count rate was on the order of 1–2 Hz. The off‐axis ion collector gives a factor of 140 increase in effective sensitivity of the mass spectrometer. The minimum detection limit of the mass spectrometer with the off‐axis detector is also determined.