Device for adjusting a control setpoint for a rotational speed, referred to as the NR speed, of at least one main rotor of a rotorcraft, said adjustment device with which a rotorcraft is provided comprising: • at least one measuring element (6, 16) for taking measurements representative of a current vertical elevation z, z', z" of said rotorcraft relative to a reference level Ref, and • means for measuring a travelling speed of said rotorcraft which make it possible to measure at least one vertical component Vz of a travelling speed of said rotorcraft relative to the air surrounding said rotorcraft, said device for adjusting said control setpoint for said NR speed comprising management means for automatically controlling said control setpoint for the NR speed according to at least two predetermined control laws NR1, NR2 that are different from one another, said at least two predetermined control laws NR1, NR2 being alternately selected during a descending flight phase of said rotorcraft at least on the basis of the measurements representative of said decreasing vertical elevation z, z', z", said management means being configured to control the control setpoint for said NR speed according to: • a first control law NR1 when said vertical elevation z, z', z" is less than a first predetermined threshold value S1, • a second control law NR2 selected so as to be different from said first control law NR1 when said vertical elevation z, z', z" is greater than a second predetermined threshold value S2 which is greater than said first predetermined threshold value S1, characterised in that said first predetermined threshold value S1 is variable according to said vertical component Vz and such that: • said first predetermined threshold value S1 is less than or equal to a first vertical elevation constant Sc1 when said vertical component Vz is greater than or equal to a vertical speed constant Vzc, and • said first predetermined threshold value S1 is greater than said first vertical elevation constant Sc1 when said vertical component Vz is less than said vertical speed constant Vzc. • at least one measuring element (6, 16) for taking measurements representative of a current vertical elevation z, z', z" of said rotorcraft relative to a reference level Ref, and • means for measuring a travelling speed of said rotorcraft which make it possible to measure at least one vertical component Vz of a travelling speed of said rotorcraft relative to the air surrounding said rotorcraft, said device for adjusting said control setpoint for said NR speed comprising management means for automatically controlling said control setpoint for the NR speed according to at least two predetermined control laws NR1, NR2 that are different from one another, said at least two predetermined control laws NR1, NR2 being alternately selected during a descending flight phase of said rotorcraft at least on the basis of the measurements representative of said decreasing vertical elevation z, z', z", said management means being configured to control the control setpoint for said NR speed according to: • a first control law NR1 when said vertical elevation z, z', z" is less than a first predetermined threshold value S1, • a second control law NR2 selected so as to be different from said first control law NR1 when said vertical elevation z, z', z" is greater than a second predetermined threshold value S2 which is greater than said first predetermined threshold value S1, characterised in that said first predetermined threshold value S1 is variable according to said vertical component Vz and such that: • said first predetermined threshold value S1 is less than or equal to a first vertical elevation constant Sc1 when said vertical component Vz is greater than or equal to a vertical speed constant Vzc, and • said first predetermined threshold value S1 is greater than said first vertical elevation constant Sc1 when said vertical component Vz is less than said vertical speed constant Vzc. • a first control law NR1 when said vertical elevation z, z', z" is less than a first predetermined threshold value S1, • a second control law NR2 selected so as to be different from said first control law NR1 when said vertical elevation z, z', z" is greater than a second predetermined threshold value S2 which is greater than said first predetermined threshold value S1, characterised in that said first predetermined threshold value S1 is variable according to said vertical component Vz and such that: • said first predetermined threshold value S1 is less than or equal to a first vertical elevation constant Sc1 when said vertical component Vz is greater than or equal to a vertical speed constant Vzc, and • said first predetermined threshold value S1 is greater than said first vertical elevation constant Sc1 when said vertical component Vz is less than said vertical speed constant Vzc. • said first predetermined threshold value S1 is less than or equal to a first vertical elevation constant Sc1 when said vertical component Vz is greater than or equal to a vertical speed constant Vzc, and • said first predetermined threshold value S1 is greater than said first vertical elevation constant Sc1 when said vertical component Vz is less than said vertical speed constant Vzc.