A motorcycle height adjustment device capable of precisely estimating a weight applied to a motorcycle, comprising: a suspension device (21, 22) comprising: a spring (110, 210) configured to be disposed between a body of the motorcycle and a wheel (2, 3); and a relative position alteration device (140, 240) with a support member (141, 241) and a hydraulic jack (143, 243), the support member (141, 241) supporting one end of the spring (110, 210) and being configured to move relative to the hydraulic jack (143, 243) toward another one end of the spring (110, 210) to change a length of the spring (110, 210); a control unit configured to determine a target movement amount for the support member (141, 241) based on a predetermined interrelation between a weight applied to the motorcycle and the target movement amount so that the target movement amount is increased as the weight increases, and to control a movement amount of the support member (141, 241) so that an actual movement amount of the support member (141, 241) reaches the target movement amount; and a weight estimation unit configured to estimate the weight, characterised in that the weight estimation unit is configured to estimate the weight based on a length of the suspension device (21, 22) and the movement amount of the support member (141, 241), wherein the control unit is configured to determine the target movement amount based on a predetermined temporary value of the weight, wherein the weight estimation unit is configured to increase the temporary value, even when the actual movement amount has reached the target movement amount, when an actual length of the suspension device (21, 22) does not reach a target length, wherein the control unit is configured to perform a control so that, when the weight estimation unit has increased the temporary value, the actual movement amount reaches the target movement amount that is set anew and that corresponds to the temporary value that is set anew, and wherein the weight estimation unit is configured to estimate as the weight the temporary value when the actual length has finally reached the target length. a suspension device (21, 22) comprising: a spring (110, 210) configured to be disposed between a body of the motorcycle and a wheel (2, 3); and a relative position alteration device (140, 240) with a support member (141, 241) and a hydraulic jack (143, 243), the support member (141, 241) supporting one end of the spring (110, 210) and being configured to move relative to the hydraulic jack (143, 243) toward another one end of the spring (110, 210) to change a length of the spring (110, 210); a spring (110, 210) configured to be disposed between a body of the motorcycle and a wheel (2, 3); and a relative position alteration device (140, 240) with a support member (141, 241) and a hydraulic jack (143, 243), the support member (141, 241) supporting one end of the spring (110, 210) and being configured to move relative to the hydraulic jack (143, 243) toward another one end of the spring (110, 210) to change a length of the spring (110, 210); a control unit configured to determine a target movement amount for the support member (141, 241) based on a predetermined interrelation between a weight applied to the motorcycle and the target movement amount so that the target movement amount is increased as the weight increases, and to control a movement amount of the support member (141, 241) so that an actual movement amount of the support member (141, 241) reaches the target movement amount; and a weight estimation unit configured to estimate the weight, characterised in that the weight estimation unit is configured to estimate the weight based on a length of the suspension device (21, 22) and the movement amount of the support member (141, 241), wherein the control unit is configured to determine the target movement amount based on a predetermined temporary value of the weight, wherein the weight estimation unit is configured to increase the temporary value, even when the actual movement amount has reached the target movement amount, when an actual length of the suspension device (21, 22) does not reach a target length, wherein the control unit is configured to perform a control so that, when the weight estimation unit has increased the temporary value, the actual movement amount reaches the target movement amount that is set anew and that corresponds to the temporary value that is set anew, and wherein the weight estimation unit is configured to estimate as the weight the temporary value when the actual length has finally reached the target length.