WE CLAIM:
1. A spunbonded nonwoven fabric comprising thermoplastic
fibers, wherein the thermoplastic fiber is a conjugate
fiber in which a low-melting polymer having a melting point
lower by 10 to 140°C than that of a high-melting polymer is
provided around the high-melting polymer, the spunbonded
nonwoven fabric has a non-pressure bonding portion having
an apparent density of 0.20 to 0.60 g/cm , when a long axis
length of a fiber cross section of the non-pressure bonding
portion is a and a short axis length thereof is b, a fiber
flatness a/b is 1.5 to 5, and an air permeability satisfies
[formula 1]:
[air permeability (cc/cm *sec)] £ 520 × exp(-0.0236 × [basis weight (g/m )] - 2.85 × [apparent density (g/cm )])... [formula 1].
2. The spunbonded nonwoven fabric according to claim 1, wherein a compression bonding ratio of the spunbonded nonwoven fabric is 5 to 40%.
3. The spunbonded nonwoven fabric according to claim 1 or 2, wherein the basis weight of the spunbonded nonwoven fabric is 10 to 150 g/m .
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4. The spunbonded nonwoven fabric according to any one of claims 1 to 3, wherein a single fiber fineness of the thermoplastic fiber is 0.5 to 3 dtex.
5. The spunbonded nonwoven fabric according to any one of claims 1 to 4, wherein the thermoplastic fiber is a polyester fiber.
6. A separation membrane support comprising the spunbonded nonwoven fabric according to any one of claims 1 to 5.
7. A production method for a spunbonded nonwoven fabric comprising the following steps (a) to (c) sequentially in this order:

(a) spinning, from a spinneret having a rectangular discharge orifice having an aspect ratio (long side length/short side length) of 1.6 to 8, a conjugate fiber in which a low-melting polymer having a melting point lower by 10 to 140°C than that of a high-melting polymer is provided around the high-melting polymer;
(b) stretching the spun conjugate fibers by suction flow using a high-speed suction gas and collecting the fibers on a moving net conveyor to form the collected fibers into a nonwoven web; and
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(c) partially thermally bonding the obtained nonwoven web at a temperature lower by 5 to 80°C than the melting point of the low-melting polymer.
8. The production method for a spunbonded nonwoven fabric according to claim 7, wherein the conjugate fiber of the step (a) is a polyester fiber.
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