  The field-free and highly efficient diodes with the nonreciprocity of
supercurrent are believed to be the core block of the superconducting computing
devices without dissipation. In this paper, we propose a Josephson diode based
upon altermagnets with the vanishing net macroscopic magnetization. The
nonreciprocity of supercurrent can be realized without applying any external
magnetic field or ferromagnetic exchange field, which can avoid the magnetic
cross-talk between the basic elements of the devices. The high efficiency
exceeding $40\%$ can be obtained and the efficiency shows the high stability
when the structure parameters are changed. The diode efficiency is
antisymmetric about the relative orientation angle of the superconducting
leads, so that its sign can easily be inverted by adjusting the relative
orientation angle. The symmetries satisfied by the current-phase difference
relations and the diode efficiency are analyzed by considering the
transformations of the junctions under the time-reversal, the spin-rotation and
the mirror reflection operations. The high efficiency and the high stability of
the Josephson diode effect in our junctions provide the possibility for the
design of the field-free dissipationless diode devices.
