The magnetic field-vs.-temperature phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4 is presented. There appears around TA=10K at H=0 a second-order phase transition from a normal metallic state to another metallic state associated with an antiferromagnetic-like magnetic-order. When the magnetic fields are applied normal to the conducting plane, TA shifts to lower temperatures, and goes to zero at HA(T=0)=23T. The inside region of the phase boundary encompassed by TA and HA is divided into two states by HB(=7T around 1K; the reversible and irreversible states of both the magnetoresistance and magnetization for H<B(T) and H>HB(T), respectively. The third characteristic field HC(T)(=10T at 1K) defined as the magnetic field where the magnetoresistance takes a maximum is located slightly above HB(T). Discussion are made on this phase diagram.