A straight tube ultrasonic flow meter comprising: a measuring unit disposed within a measurement space of a casing, the measuring unit having a tubular body for measurement as a straight tube through which a fluid for measurement flows and a pair of oscillators (22a, 22b) disposed around an outer circumference of the tubular body at a predetermined interval along an axial direction, the measuring unit capable of measuring a flow rate of the fluid by obtaining a flow velocity of the fluid for measurement based on a difference in propagation time of ultrasonic wave from both directions detected between the oscillators (22a, 22b), wherein the straight tube ultrasonic flow meter further comprises a measuring unit protecting joint (30, 30A) which is fixed to the casing and which is interposed in a connection to an external pipe, wherein the measuring unit protecting joint (30, 30A) includes: a protecting joint main body, a fixation ring configured to engage with an inside screw section provided in the wall surface of the casing and prevent the protecting joint main body (31 A) from moving in the axial direction, and a contracted section disposed on a downstream side of the oscillators (22a, 22b) that is provided removably by inserting a contraction providing member into a downstream outlet of the tubular body, wherein the protecting joint main body (31, 31A, 31 B) includes a pipe supporting section in which the tubular body for measurement is inserted to a predetermined position, and that supports the tubular body by a portion in vicinity of an end portion, wherein, the tubular body and the external pipe are connected so as to form a fluid flow channel at the pipe supporting section, wherein the measuring unit protecting joint (30, 30A) restricts movement of the tubular body of the measuring unit to a central side in the axial direction and rotation of the measuring unit with respect to the casing through the pipe supporting section, wherein the protecting joint main body (31, 31A, 31 B) further includes a casing opening insertion section that is inserted into and assembled with an opening provided in a wall surface of the casing, the opening having a cross-sectional shape for restricting rotation, the casing opening insertion section having a cross-sectional shape identical to the cross-sectional shape for restricting rotation; a contact surface that defines an insertion position of the casing opening insertion section to a central side in the axial direction, and wherein the contracted section increases a pressure within the tubular body upstream of the contracted section by decreasing a cross-sectional area of the flow channel of the tubular body at the contracted section so as to crush and remove small air bubbles that gather around an inner wall surface of the tubular body. a measuring unit disposed within a measurement space of a casing, the measuring unit having a tubular body for measurement as a straight tube through which a fluid for measurement flows and a pair of oscillators (22a, 22b) disposed around an outer circumference of the tubular body at a predetermined interval along an axial direction, the measuring unit capable of measuring a flow rate of the fluid by obtaining a flow velocity of the fluid for measurement based on a difference in propagation time of ultrasonic wave from both directions detected between the oscillators (22a, 22b), wherein the straight tube ultrasonic flow meter further comprises a measuring unit protecting joint (30, 30A) which is fixed to the casing and which is interposed in a connection to an external pipe, wherein the measuring unit protecting joint (30, 30A) includes: a protecting joint main body, a fixation ring configured to engage with an inside screw section provided in the wall surface of the casing and prevent the protecting joint main body (31 A) from moving in the axial direction, and a contracted section disposed on a downstream side of the oscillators (22a, 22b) that is provided removably by inserting a contraction providing member into a downstream outlet of the tubular body, wherein the protecting joint main body (31, 31A, 31 B) includes a pipe supporting section in which the tubular body for measurement is inserted to a predetermined position, and that supports the tubular body by a portion in vicinity of an end portion, wherein, the tubular body and the external pipe are connected so as to form a fluid flow channel at the pipe supporting section, wherein the measuring unit protecting joint (30, 30A) restricts movement of the tubular body of the measuring unit to a central side in the axial direction and rotation of the measuring unit with respect to the casing through the pipe supporting section, wherein the protecting joint main body (31, 31A, 31 B) further includes a casing opening insertion section that is inserted into and assembled with an opening provided in a wall surface of the casing, the opening having a cross-sectional shape for restricting rotation, the casing opening insertion section having a cross-sectional shape identical to the cross-sectional shape for restricting rotation; a contact surface that defines an insertion position of the casing opening insertion section to a central side in the axial direction, and wherein the contracted section increases a pressure within the tubular body upstream of the contracted section by decreasing a cross-sectional area of the flow channel of the tubular body at the contracted section so as to crush and remove small air bubbles that gather around an inner wall surface of the tubular body.