A drive source switching system comprising: a drive source switching mechanism (22, 23) that is configured to switch, between a first drive source and a second drive source of a marine propulsion device, the drive sources (16, 17) generating a drive force to be transmitted to a propeller shaft of the marine propulsion device; and a controller configured or programmed to control the drive source switching mechanism (22, 23) to switch the drive source between the first drive source and the second drive source, wherein when switching the drive source between the first drive source and the second drive source fails, the controller is configured to temporarily interrupt and/or to move back the switching of the drive source between the first drive source and the second drive source, characterised in that the drive source switching mechanism (22, 23) comprises a first driving member configured to transmit a drive force generated by the first drive source; a second driving member (37, 38) configured to transmit a drive force generated by the second drive source; a first driven member that is movable in an axial direction of the propeller shaft, engageably with the first driving member; and a second driven member that is movable in the axial direction of the propeller shaft, engageably with the second driving member (37, 38); and the controller is configured or programmed to control a movement of the first driven member, wherein the first driven member is configured to transmit the drive force generated by the first drive source to the propeller shaft when engaged with the first driving member, the second driven member is configured to transmit the drive force generated by the second drive source to the propeller shaft when engaged with the second driving member (37, 38), and in a case that the controller operates to move the first driven member at a first position where the first driven member is not engaged with the first driving member, in the axial direction of the propeller shaft toward the first driving member, the controller operates to interrupt a movement of the first driving member and/or to move back the first driven member in a direction of the first position when the first driven member fails to be engaged with the first driving member. a drive source switching mechanism (22, 23) that is configured to switch, between a first drive source and a second drive source of a marine propulsion device, the drive sources (16, 17) generating a drive force to be transmitted to a propeller shaft of the marine propulsion device; and a controller configured or programmed to control the drive source switching mechanism (22, 23) to switch the drive source between the first drive source and the second drive source, wherein when switching the drive source between the first drive source and the second drive source fails, the controller is configured to temporarily interrupt and/or to move back the switching of the drive source between the first drive source and the second drive source, characterised in that the drive source switching mechanism (22, 23) comprises a first driving member configured to transmit a drive force generated by the first drive source; a second driving member (37, 38) configured to transmit a drive force generated by the second drive source; a first driven member that is movable in an axial direction of the propeller shaft, engageably with the first driving member; and a second driven member that is movable in the axial direction of the propeller shaft, engageably with the second driving member (37, 38); and the controller is configured or programmed to control a movement of the first driven member, wherein the first driven member is configured to transmit the drive force generated by the first drive source to the propeller shaft when engaged with the first driving member, the second driven member is configured to transmit the drive force generated by the second drive source to the propeller shaft when engaged with the second driving member (37, 38), and in a case that the controller operates to move the first driven member at a first position where the first driven member is not engaged with the first driving member, in the axial direction of the propeller shaft toward the first driving member, the controller operates to interrupt a movement of the first driving member and/or to move back the first driven member in a direction of the first position when the first driven member fails to be engaged with the first driving member.