Studies of internal flow and discharged liquid jets from the nozzle are sometimes carried out under steady injection conditions. Although steady injection data can be helpful for the study of internal flow in the nozzle, fuel injection is carried out under a transient injection scheme, which gives in-nozzle cavitation phenomena a transient characteristic. This difference raises some questions regarding the applicability of steady injection data to the transient injection process. In this study, high-speed visualization of cavitation in a rectangular plain-orifice nozzle and discharged liquid jet was carried out under steady and transient injection conditions in order to examine the applicability of steady injection data to the transient injection process. The cavitation length and discharged liquid jet angle data from the transient injection is used to investigate transient cavitation development in the macro scale, while X-ray phase contrast imaging of in-nozzle cavitation was carried out to clarify the morphology of cavitation inception in the micro scale. From the study, we clarified the applicability of steady injection data to transient injection processes. Correlations obtained from steady injection data can be used to predict cavitation growth and the discharged liquid jet angle during the transient injection process where the duration of the flow rate increase is much longer than the time scale of flow development in the nozzle. High-speed X-ray phase contrast imaging revealed two kinds of heterogeneous nucleation processes of cavitation inception induced by bubble nuclei in the bulk flow or the nozzle wall surface. Both are governed by the turbulent flow structure in the nozzle.