The Si atom density at the 3p(2) P-3(2) level in electron cyclotron resonance SiH4/H-2 and SiH4/Ar plasmas was measured as a function of total pressure from 0.6 to 3.3 Pa at a microwave power of 400 W and a SiH4 flow rate of 6 seem using ultraviolet absorption spectroscopy. Parent SiH4 molecules in SiH4/H-2 and SiH4/Ar plasmas were found to be considerably dissociated using infrared diode laser absorption spectroscopy. The Si atom density in the SiH4/H-2 plasma was larger than that in the SiH4/Ar plasma, which was quite different from that in the capacitively coupled rf SiH4 plasmas. Behaviors of the Si atom density are discussed on the basis of the rate equation for Si atoms. It was concluded from a comparison of the behavior of the density and generation rate of Si atoms that the diffusion loss was comparable to the reaction loss in the removal process of Si atoms in the case of H-2 dilution, while the reaction loss was dominant in the case of Ar dilution.