The object of this study is to investigate experimentally effects of EGR gas on the ignition delay and the combustion, in a diesel spray using a rapid compression-expansion machine. EGR gas has CO2, H2O and N2. Therefore, we also research the influence of each gas on the ignition and the combustion individually. The results show that changes of ignition delay are few until the EGR rate of 20 %. When the EGR rate reaches 25 %, ignition delay increases drastically. The gradient of temperature against the ignition delay in arrhenius plot changes at 900 K. That phenomenon is independent of an ambient gas composition. Additions of CO2 and H2O to air prevent to lengthen ignition delay because of a low oxygen condition. Additions of N2 to air have longer ignition delay than EGR condition. And the reduction in the ambient gas oxygen concentration tends to make the two-stage combustion vigorous.