The magnetic properties of the single crystalline Mn2AS4 (A=Si and Ge) with an olivine structure, which are the uniaxially anisotropic antiferromagnets (the b axis as an easy axis), were investigated. Near the Néel temperature, both compounds exhibit the contrastive magnetic responses along the c axis, namely, the spontaneous weak ferromagnetism in A=Si and the significant enhancement of the differential susceptibility (dM/dH) under the small magnetic field in A=Ge. When A=Ge, we also observed the evolution of dM/dH along the a axis at low temperatures. We discuss these phenomena on the basis of the magnetic field-temperature (H-T) phase diagram with the spin-flop bicritical point (HBP,TBP). The role of the thermal or quantum fluctuation was stressed.