A flow meter includes a transparent holder with an inlet and an outlet connected to a first, second and third fastening portions through a first, second, third and fourth connection ribs, a first and second fastening assemblies screwed respectively with the inlet and outlet, a butting member located at one end of the transparent holder, a flow scale plate located on a recess of the transparent holder, and a regulation member coupled to the inlet. The three fastening portions with respectively a first, second and third fastening hole are integrally formed with the inlet and outlet. Through the first and second fastening holes, the transparent holder is firmly fastened to a machine. Thus hydraulic pressure generated in the inlet and outlet does not cause loosening due to screw fastening of the three fastening portions. The structural strength of the transparent holder is increased and assembly is easier.