A surgical instrument comprising: at a distal portion, a tool head having longitudinal and lateral degrees of freedom; and at a proximal portion, a first manipulator to control the longitudinal degrees of freedom and a second manipulator to control the lateral degrees of freedom; wherein the tool head includes a movable portion having longitudinal degrees of freedom and a base portion that defines a lateral orientation of the tool head, the movable portion and the first manipulator are connected by a first connecting structure, the base portion and the second manipulator are connected by a second connecting structure, the first connecting structure includes: a first conversion mechanism that converts linear motion from the first manipulator to rotational motion; a first shaft that is to be rotated by the first conversion mechanism; a universal joint provided at a distal end portion of the first shaft; a second conversion mechanism that is connected to the universal joint to convert rotational motion into linear motion; and a crank provided at a distal end portion of the second conversion mechanism and connected to a proximal end portion of the movable portion; the second connecting structure is configured to transmit rotational motion of the second manipulator, and provided with: a tubular shaft in which the first shaft is coaxially encompassed and a pinion is formed at a distal portion; and a rack base portion being linked to the base portion, on which a rack to be meshed with the pinion is formed in an approximate semi-circular shape; and an intersection of shaft centers of two pivot shafts of the universal joint is positioned on an axial center of rotational axis of the rack. at a distal portion, a tool head having longitudinal and lateral degrees of freedom; and at a proximal portion, a first manipulator to control the longitudinal degrees of freedom and a second manipulator to control the lateral degrees of freedom; wherein the tool head includes a movable portion having longitudinal degrees of freedom and a base portion that defines a lateral orientation of the tool head, the movable portion and the first manipulator are connected by a first connecting structure, the base portion and the second manipulator are connected by a second connecting structure, the first connecting structure includes: a first conversion mechanism that converts linear motion from the first manipulator to rotational motion; a first shaft that is to be rotated by the first conversion mechanism; a universal joint provided at a distal end portion of the first shaft; a second conversion mechanism that is connected to the universal joint to convert rotational motion into linear motion; and a crank provided at a distal end portion of the second conversion mechanism and connected to a proximal end portion of the movable portion; the second connecting structure is configured to transmit rotational motion of the second manipulator, and provided with: a tubular shaft in which the first shaft is coaxially encompassed and a pinion is formed at a distal portion; and a rack base portion being linked to the base portion, on which a rack to be meshed with the pinion is formed in an approximate semi-circular shape; and an intersection of shaft centers of two pivot shafts of the universal joint is positioned on an axial center of rotational axis of the rack. <EOS>