Injection device for injecting liquid medicine to a patient, comprising: - a housing, - a static structure comprising a first gear member, said static structure as a whole being fixed relative to the housing, - a dynamic structure comprising a medicine container, means (6, 7, 8) for pushing liquid medicine out of said medicine container for its injection to a patient, and a second gear member engaged with the first gear member, said dynamic structure as a whole being moveable relative to the housing, - a drive member for driving one of the first and second gear members (14, 13) so as to cause the dynamic structure to move relative to the static structure along a predetermined direction due to the engagement between the first and second gear members (14, 13), and - clutch means (23, 25, 28) for maintaining engagement between the first and second gear members (14, 13) in normal condition of the injection device, and for allowing disengagement of the first and second gear members (14, 13) upon a shock received by the injection device along the predetermined direction, said clutch means comprising: - means for allowing the other of the first and second gear members (14, 13) to move away from said one of the first and second gear members (14, 13) and thus disengage from said one of the first and second gear members (14, 13), - a blocking member which, in a predetermined position, prevents the other of the first and second gear members (14, 13) from moving away from said one of the first and second gear members (14, 13), and - a return spring for maintaining the blocking member in the predetermined position, the force of said return spring _being overcomeable by the other of the first and second gear members (14, 13) upon a shock received by the injection device along the predetermined direction and moving the other of the first and second gear members (14, 13) away from said one of the first and second gear members. - a housing, - a static structure comprising a first gear member, said static structure as a whole being fixed relative to the housing, - a dynamic structure comprising a medicine container, means (6, 7, 8) for pushing liquid medicine out of said medicine container for its injection to a patient, and a second gear member engaged with the first gear member, said dynamic structure as a whole being moveable relative to the housing, - a drive member for driving one of the first and second gear members (14, 13) so as to cause the dynamic structure to move relative to the static structure along a predetermined direction due to the engagement between the first and second gear members (14, 13), and - clutch means (23, 25, 28) for maintaining engagement between the first and second gear members (14, 13) in normal condition of the injection device, and for allowing disengagement of the first and second gear members (14, 13) upon a shock received by the injection device along the predetermined direction, - means for allowing the other of the first and second gear members (14, 13) to move away from said one of the first and second gear members (14, 13) and thus disengage from said one of the first and second gear members (14, 13), - a blocking member which, in a predetermined position, prevents the other of the first and second gear members (14, 13) from moving away from said one of the first and second gear members (14, 13), and - a return spring for maintaining the blocking member in the predetermined position, the force of said return spring _being overcomeable by the other of the first and second gear members (14, 13) upon a shock received by the injection device along the predetermined direction and moving the other of the first and second gear members (14, 13) away from said one of the first and second gear members.