A collecting device of biological fluids for medical use, comprising: - a rigid container internally defining at least a first chamber having a fixed volume and exhibiting a box conformation having at least a bottom wall, at least a lateral wall emerging from the bottom wall and at least a closing portion located at a top of the lateral wall, the walls defining the volume of the first chamber wherein at least a ventilation line is provided, predisposed such as to create a connection between the first chamber and the external environment; - an auxiliary container having at least a deformable portion internally defining a second chamber having a variable volume according to a position of the deformable portion; - at least a passage able to set the first and the second chamber (2a, 3a) in fluid communication; - at least an inlet port connected with at least one of the containers; - at least an outlet port connected with at least one of the containers; - a supply channel connected with the inlet port; - a discharge channel connected with the outlet port; - a bypass channel which exhibits a first end connected with the supply channel and a second end connected with the discharge channel for bypassing the fluid collecting device; and - a fluid intercept member operating on the bypass channel and at least one from among a fluid intercept member operating on the supply channel and an intercept member operating on the discharge channel; and only the auxiliary container comprises both the inlet port and the outlet port, characterized in that it comprises a part emerging from a portion of lateral wall of the rigid container, with reference to an operating configuration in which the lateral wall extends from below in an upwards direction, the part developing inferiorly with respect to the first chamber and in prolongation with respect to the lateral wall; and wherein the auxiliary container comprises a rigid wall formed by a portion of the lateral wall of the rigid container and by the inferiorly-extending part with respect to the first chamber; and a perimetrally flexible wall fixed sealedly on the rigid wall of the auxiliary container. - a rigid container internally defining at least a first chamber having a fixed volume and exhibiting a box conformation having at least a bottom wall, at least a lateral wall emerging from the bottom wall and at least a closing portion located at a top of the lateral wall, the walls defining the volume of the first chamber wherein at least a ventilation line is provided, predisposed such as to create a connection between the first chamber and the external environment; - an auxiliary container having at least a deformable portion internally defining a second chamber having a variable volume according to a position of the deformable portion; - at least a passage able to set the first and the second chamber (2a, 3a) in fluid communication; - at least an inlet port connected with at least one of the containers; - at least an outlet port connected with at least one of the containers; - a supply channel connected with the inlet port; - a discharge channel connected with the outlet port; - a bypass channel which exhibits a first end connected with the supply channel and a second end connected with the discharge channel for bypassing the fluid collecting device; and - a fluid intercept member operating on the bypass channel and at least one from among a fluid intercept member operating on the supply channel and an intercept member operating on the discharge channel; and only the auxiliary container comprises both the inlet port and the outlet port, characterized in that it comprises a part emerging from a portion of lateral wall of the rigid container, with reference to an operating configuration in which the lateral wall extends from below in an upwards direction, the part developing inferiorly with respect to the first chamber and in prolongation with respect to the lateral wall; and wherein the auxiliary container comprises a rigid wall formed by a portion of the lateral wall of the rigid container and by the inferiorly-extending part with respect to the first chamber; and a perimetrally flexible wall fixed sealedly on the rigid wall of the auxiliary container.