In a semiconductor device comprising a first bipolar transistor and a second bipolar transistor having different voltages formed on a semiconductor substrate made by forming an epitaxial layer on a silicon substrate, in an upper part of the silicon substrate the first bipolar transistor has an N⁺-type first embedded diffusion layer having an impurity concentration higher than that of the epitaxial layer and the second bipolar transistor has an N-type second embedded diffusion layer having a lower impurity concentration and a deeper diffusion layer depth than the first embedded diffusion layer, whereby a high speed bipolar transistor and a high voltage bipolar transistor are formed on the same substrate.