Recently, multiple locomotion gaits of snakes have been realized by snake robots. A snake robot would be applied to going into a narrow space to investigate the inside of a long and slim pipe and to executing some simple work by using its slim body. In this paper, we achieve a new locomotion mode that propels a snake robot the inside of a pipe with helical rolling motion, in addition to previously achieved locomotion modes. A structure of the snake robot is constructed by connecting many units in series. The unit has a pitch axis, a yaw axis and two passive wheels. A shape of the robot is calculated by using mathematical continuum curve model. In addition, we present a method that the shape of snake robot is adjusted to a diameter of a pipe automatically. Moreover, we indicate a possibility of a new locomotion mode which conduct helical rolling motion with conical spiral form in order to improve a position of a camera while the snake robot investigates the inside of a pipe.