The dependence of the formation of surface roughness in Si on the incident angle of the Cs+ primary ion in secondary ion mass spectrometry is reported. No ripples formed in the analytical crater bottom when the primary ion incident angle was from 0-degree to 30-degrees for sputtered depths of less than 4 mum, but ripples were observed when the incident angle was from 45-degrees to 75-degrees. The depth of ripple formation became shallower with the incident angle increasing. Cross sections of ripples observed by a scanning electron microscope and a transmission electron microscope suggest that ripples grow by forming facets from those plane faces with the largest sputtering rates so that the shapes of the facets remain constant during sputtering.