The diffusion of a negatively buoyant jet that issued horizontally from a pipe outlet into shallow water and fell onto the horizontal bed was investigated experimentally. The densimetric Froude number of the jets are from 6.7 to 17.0. The velocity profile and the temperature profile of the freely falling jet and those of the horizontally spreading jet on the bed floor are emerged.Those profiles keep similarity in shape in each zone. As the distance s from the outlet along the axis of the jet increases, the maximum velocity of the profile decreases in proportion to s^-1 or less in the freely falling zone. Meanwhile the maximum velocity decreases in proportion to s^-1.5in the horizontally spreading zone.When the jet impinges onto the bed floor, the freely falling jet transforms smoothly into the horizontally spreading jet, but the entrainment of jet pause for a while.