It has been recently reported that the porosity and the density, etc. in the surficial marine sediment vary largely with the depth. The characterisitics of acoustic wave reflection from such transition layers of the surficial marine sediment seem to be very different from that of uniform sediments. In this study, the incident angle dependence of the reflection coefficient for the transition layer of the surficial marine sediment model is calculated using OASES (Biot–Stoll model). The effects of the transition layer and the frequency dependence of the reflection coefficient characteristics are investigated. Next, the characteristics of the incident angle for the reflection coefficient are measured in water tank and in situ. Beach sand (fine sand) is used for water tank measurements, and in situ measurements are done in a very fine sand bottom. The operating frequency is 150 kHz. The incident angle are 0, 20, 50, and 60 deg. The measured results are compared with the calculated results using OASES (Biot–Stoll model), and the effects of the transition layer on the characteristics of the reflection coefficient are investigated.