A method for manufacturing a measurement pipeline portion of an ultrasonic flow meter by blow molding that uses a mold divisible into a plurality of pieces (Ma, Mb) having an inner mold which forms an outer surface of the measurement pipeline portion, wherein the measurement pipeline portion of an ultrasonic flow meter includes a straight pipe-shaped pipe body, a fluid inlet portion disposed in one end portion of the pipe body, a fluid outlet portion disposed in the other end portion of the pipe body, and a pair of ultrasonic wave input-output portions (14a, 14b) for attaching an ultrasonic wave transmission-reception unit transmitting and receiving an ultrasonic beam to and from an inside of the pipe body, the method comprising: a step of opening the mold, storing a tubular parison as a soft-melted thermoplastic synthetic resin material in the mold, and both tips of the parison being sandwiched between the molds (Ma, Mb); a step of molding the measurement pipeline portion by closing the mold, expanding the parison by injecting a gas into the parison, via an air pipe which is attached to at least one of the fluid inlet portion and the fluid outlet portion and bringing an outer surface of the parison into close contact with the inner mold of the mold; a step of taking out the solidified measurement pipeline portion by opening the mold after cooling the parison; and a step of cutting end portions of the fluid inlet portion and the fluid outlet portion of the measurement pipeline portion, characterized in that the measurement pipeline portion is manufactured by integral blow molding, both cut ends of the pipe body become the fluid inlet portion and the fluid outlet portion continuous with the same diameter, the pair of ultrasonic wave input-output portions (14a,14b) are formed in a sealed and bulging manner on opposite side surfaces of the pipe body positioned obliquely to a longitudinal center line of the pipe body as if a column body is obliquely embedded in the pipe body, planar wall surfaces (15a, 15b) directly facing each other for attaching the ultrasonic wave transmission-reception unit (Sa, Sb) correspond to both end surfaces of the column body in the pair of the ultrasonic wave input-output portions (14a, 14b) respectively, acute angle parts of outlines of the column body in the ultrasonic wave input-output portions (14a,14b) are ridgeline portions (16a,16b) which are chamfered, rounded and smooth, and the inner mold of the molds (Ma, Mb) is processed such that the acute angle parts of the ultrasonic wave input-output portions (14a,14b) become the smooth ridgeline portions (16a,16b). a step of opening the mold, storing a tubular parison as a soft-melted thermoplastic synthetic resin material in the mold, and both tips of the parison being sandwiched between the molds (Ma, Mb); a step of molding the measurement pipeline portion by closing the mold, expanding the parison by injecting a gas into the parison, via an air pipe which is attached to at least one of the fluid inlet portion and the fluid outlet portion and bringing an outer surface of the parison into close contact with the inner mold of the mold; a step of taking out the solidified measurement pipeline portion by opening the mold after cooling the parison; and a step of cutting end portions of the fluid inlet portion and the fluid outlet portion of the measurement pipeline portion,