A controller for controlling an operation of transporting a sheet by controlling a first driving device (71,73) which drives a first roller to rotate and a second driving device (81,83) which drives a second roller to rotate, in a transport mechanism performing the operation of transporting the sheet by rotating the first roller and the second roller which are arranged apart from each other along a transporting path of the sheet, with the first roller downstream of the second roller, characterised in that the controller comprises: a first estimating unit configured to estimate a reaction force R1 which acts on the first roller in a case that the first roller is driven to rotate by the first driving device (71,73); a second estimating unit configured to estimate a reaction force R2 which acts on the second roller in a case that the second roller is driven to rotate by the second driving device (81,83); a first calculating unit configured to calculate a control input U1 in accordance with a deviation between a target state quantity, and a state quantity of the sheet (Z1 + Z2)/2 corresponding to the sum (Z1 + Z2) of a state quantity Z1 and a state quantity Z2, by using the state quantity Z1 concerning a rotary motion of the first roller, and the state quantity Z2 concerning a rotary motion of the second roller, the state quantities Z1 and Z2 being measured respectively by first and second measuring devices (75,77,85,87); a second calculating unit configured to calculate a control input U2 in accordance with a deviation between a target tension, and a tension of the sheet (R1 - R2)/2 corresponding to the difference (R1 - R2) between the reaction force R1 estimated by the first estimating unit and the reaction force R2 estimated by the second estimating unit; a first drive controller (217,218) configured to input, to the first driving device (71,73), a control signal in accordance with the sum (U1 + U2) of the control input U1 and the control input U2; and a second drive controller (227,228) configured to input, to the second driving device (81,83), a control signal in accordance with the difference (U1 - U2) between the control input U1 and the control input U2. a first estimating unit configured to estimate a reaction force R1 which acts on the first roller in a case that the first roller is driven to rotate by the first driving device (71,73); a second estimating unit configured to estimate a reaction force R2 which acts on the second roller in a case that the second roller is driven to rotate by the second driving device (81,83); a first calculating unit configured to calculate a control input U1 in accordance with a deviation between a target state quantity, and a state quantity of the sheet (Z1 + Z2)/2 corresponding to the sum (Z1 + Z2) of a state quantity Z1 and a state quantity Z2, by using the state quantity Z1 concerning a rotary motion of the first roller, and the state quantity Z2 concerning a rotary motion of the second roller, the state quantities Z1 and Z2 being measured respectively by first and second measuring devices (75,77,85,87); a second calculating unit configured to calculate a control input U2 in accordance with a deviation between a target tension, and a tension of the sheet (R1 - R2)/2 corresponding to the difference (R1 - R2) between the reaction force R1 estimated by the first estimating unit and the reaction force R2 estimated by the second estimating unit; a first drive controller (217,218) configured to input, to the first driving device (71,73), a control signal in accordance with the sum (U1 + U2) of the control input U1 and the control input U2; and a second drive controller (227,228) configured to input, to the second driving device (81,83), a control signal in accordance with the difference (U1 - U2) between the control input U1 and the control input U2.