To enable restarting of engine in the air without deceleration to subsonic speed by providing a restart by-pass duct which by-passes whole engine or fan.

A supersonic aircraft engine 10 provides a re-start by-pass duct 20 for by-passing at least one side of a turbojet enzine 11. The re-start by-pass duct 20 is adapted to by-pass only a fan 12 of the turbojet engine 11. By opening or closing an operated air inlet 22 of the re-start by-pass duct 20 during restraining, part of the air compressed in the air intake 1 is by-passed, and so, the inlet pressure P1 and the outlet pressure P2 can be adjusted to the pressure suitable for starting the turbojet engine 11, thus, the difficulties in restarting are eliminated and without deceleration to subsonic speed, the restarting can be conducted.