Method for pretreating plant starting material for the purpose of producing bioethanol and sugar, comprising the steps consisting in: - providing a common chamber for pretreatment of the plant material, comprising: -- at least one upstream inlet for introducing plant material to be pretreated into the common pretreatment chamber; -- at least one downstream outlet for discharging the pretreated plant material from the common pretreatment chamber; -- means for circulating the plant material from upstream to downstream; -- means (Gi) for bringing the plant material into contact with a pretreatment liquid which circulates overall, from downstream to upstream, in the opposite direction to the direction of circulation of the plant material inside said common pretreatment chamber; -- and means for recovering, on the one hand, the solid phase and, on the other hand, the liquid phase containing in particular at least a part of the pretreatment liquid; - during one period, introducing into the common pretreatment chamber, a lignocellulosic plant starting material to be pretreated, said pretreatment of which is aimed at separating the cellulose, the hemicelluloses and the lignins contained in said lignocellulosic plant starting material so as to obtain a pretreated plant material that can be hydrolysed and fermented for the production of bioethanol; - during another period, introducing into the common pretreatment chamber, a sacchariferous plant starting material to be pretreated, said pretreatment of which is aimed at extracting therefrom, by diffusion, a sugar juice for the production of sugar and of bioethanol. - providing a common chamber for pretreatment of the plant material, comprising: -- at least one upstream inlet for introducing plant material to be pretreated into the common pretreatment chamber; -- at least one downstream outlet for discharging the pretreated plant material from the common pretreatment chamber; -- means for circulating the plant material from upstream to downstream; -- means (Gi) for bringing the plant material into contact with a pretreatment liquid which circulates overall, from downstream to upstream, in the opposite direction to the direction of circulation of the plant material inside said common pretreatment chamber; -- and means for recovering, on the one hand, the solid phase and, on the other hand, the liquid phase containing in particular at least a part of the pretreatment liquid; -- at least one upstream inlet for introducing plant material to be pretreated into the common pretreatment chamber; -- at least one downstream outlet for discharging the pretreated plant material from the common pretreatment chamber; -- means for circulating the plant material from upstream to downstream; -- means (Gi) for bringing the plant material into contact with a pretreatment liquid which circulates overall, from downstream to upstream, in the opposite direction to the direction of circulation of the plant material inside said common pretreatment chamber; -- and means for recovering, on the one hand, the solid phase and, on the other hand, the liquid phase containing in particular at least a part of the pretreatment liquid; - during one period, introducing into the common pretreatment chamber, a lignocellulosic plant starting material to be pretreated, said pretreatment of which is aimed at separating the cellulose, the hemicelluloses and the lignins contained in said lignocellulosic plant starting material so as to obtain a pretreated plant material that can be hydrolysed and fermented for the production of bioethanol; - during another period, introducing into the common pretreatment chamber, a sacchariferous plant starting material to be pretreated, said pretreatment of which is aimed at extracting therefrom, by diffusion, a sugar juice for the production of sugar and of bioethanol.