The present invention provides a surface treatment method for magnesium alloys that can form a uniform, fine, and dense conversion coating on a magnesium alloy surface on which mold release agent, an oxide layer, and an alloying component (e.g., aluminum and zinc) segregation layer are potentially present, and also provides magnesium alloy members whose surface has been treated by the aforesaid surface treatment method. The surface treatment method of the present invention comprises a degreasing process to degrease the surface of the magnesium alloy, a chemically etching process to chemically etch the alloy, and a conversion treatment process to form a conversion coating. The chemical etching forms a magnesium phosphate coating having a coating weight of 10 to 2,000 mg/m2, measured as phosphorus, by bringing the surface of the magnesium alloy into contact with an aqueous solution containing a phosphoric acid-type compound.