Single giant duplex DNA(T4DNA, 166 kbp) complexed with a polycationic protein, histone H1, is trapped hy a continuous-wave infrared laser, without chemical modification of the DNA such as attachment to a micrometer-sized bead. When the DNA-H1 complex is transported from a low-salt solution (0.2 M NaCl) to a high-salt solution (2 M NaCl), its conformation changes from a compact to an elongated structure. With the reverse procedure, i.e., with the transportation from the high-salt to the low-salt environment, the elongated chain shrinks into a compact state. These results afford the direct evidence that histone H1 induces a salt-dependent transition in the higher-order structure of giant DNA molecules.