The interaction process between fast heavy ions and dense plasma was experimentally investigated. We injected 4.3-MeV/u or 6.0-MeV/u iron ions into a z-pinch-discharge helium plasma and measured the energy loss of the ions by the time of flight method. The energy loss of 4.3-MeV/u ions fairly agreed with theoretical prediction when the electron density of the target was on the order of 1018 cm−3. With increasing electron density beyond 1019 cm−3, the difference between the experiment and the theory became remarkable; the experimental energy loss was 15% larger than the theoretical value at the peak density. For 6.0-MeV/u ions, the deviation from the theory appeared even at densities below 1019 cm−3. These discrepancies indicated that density effects such as ladderlike ionization caused the enhancement of the projectile mean charge in the target.