A multi-beam apparatus for observing a sample with high resolution and high throughput and in flexibly varying observing conditions is proposed. The apparatus uses a movable collimating lens to flexibly vary the currents of the plural probe spots without influencing the intervals thereof, a new source-conversion unit to form the plural images of the single electron source and compensate off-axis aberrations of the plural probe spots with respect to observing conditions, and a pre-beamlet-forming means to reduce the strong Coulomb effect due to the primary-electron beam.