The purpose of this study is to propose a method of measuring the density of cohesive-soil ground with a split spoon sampler with built-in tubes. An optimal method was devised for measuring the density of samples taken from cohesive-soil ground with a split spoon sampler with built-in tubes. Then, a field experiment was carried out using a split spoon sampler with built-in tubes and a thin-wall sampler. The wet density, water content, dry density, and fine-grained fraction of samples by the split spoon sampler with build-in tubes were checked well with those of samples by the thin-wall sampler, proving the proposed method to be practical enough.