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, an apparent
single fiber fineness of the conjugate fiber as viewed from
the surface of the spunbonded nonwoven fabric is 0.5 dtex
or more and 10 dtex or less, when the apparent single fiber
fineness of the conjugate fiber is 0.5 dtex or more and
less than 2 dtex, an apparent density is 0.50 to 0.70 g/cm ,
and air permeability satisfies [formula 1] described below,
and when the apparent single fiber fineness of the
conjugate fiber is 2 dtex or more and 10 dtex or less, the
apparent density is 0.50 to 0.80 g/cm , and the air
permeability satisfies [formula 2] described below,
(cc/cm -sec)]
3.8 × 10 × [basis weight (g/m )]- £ [air permeability
£ 6.0 × 10 × [basis weight (g/m )] ...
[formula 1], and
2.2 × 10 × [basis weight (g/m )]- £ [air permeability
(cc/cm -sec)] £ 4.6 × 10 × [basis weight (g/m )] - ...
[formula 2].
2. The spunbonded nonwoven fabric according to claim 1,
94

wherein the apparent single fiber fineness of the conjugate fiber as viewed from the surface of the spunbonded nonwoven fabric is 0.5 dtex or more and less than 2 dtex, and a Bekk smoothness of at least one surface of the spunbonded nonwoven fabric is 1 to 10 seconds.
3. The spunbonded nonwoven fabric according to claim 1, wherein the apparent single fiber fineness of the conjugate fiber as viewed from the surface of the spunbonded nonwoven fabric is 2 dtex or more and 10 dtex or less, and a Bekk smoothness of at least one surface of the spunbonded nonwoven fabric is 3 to 20 seconds.
4. The spunbonded nonwoven fabric according to any one of claims 1 to 3, wherein the basis weight of the spunbonded nonwoven fabric is 10 to 150 g/m2.
5. The spunbonded nonwoven fabric according to any one of claims 1 to 4, wherein the thermoplastic fiber is a polyester fiber.
6. The spunbonded nonwoven fabric according to any one of claims 1 to 5, wherein the thermoplastic fiber has a movable amorphous content of 35 to 50%.
95

7. A separation membrane support comprising the spunbonded nonwoven fabric according to any one of claims 1 to 6.
8. A production method for a spunbonded nonwoven fabric comprising the following steps (a) to (d) sequentially in this order:

(a) spinning, from a spinneret, 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,
(c) thermally bonding the obtained nonwoven web with a pair of upper and lower flat rolls at a temperature lower by 65 to 95°C than the melting point of the low-melting polymer, and
(d) subsequently thermally bonding with the pair of upper and lower flat rolls at a temperature lower by 5 to 60°C than the melting point of the low-melting polymer.
9. The production method for a spunbonded nonwoven
fabric according to claim 8, wherein the spinneret of the
96

step (a) is a spinneret having a round discharge orifice.
10. The production method for a spunbonded nonwoven fabric according to claim 8, wherein the spinneret of the step (a) is a spinneret having a discharge orifice having an aspect ratio (long side length/short side length) of 1.6 to 8.
11. The production method for a spunbonded nonwoven fabric according to any one of claims 8 to 10, wherein the movable amorphous content of the spunbonded nonwoven fabric obtained in the step (c) is 40 to 55%.
12. The production method for a spunbonded nonwoven fabric according to any one of claims 8 to 11, wherein the movable amorphous content of the spunbonded nonwoven fabric obtained in the step (d) is 35 to 50%.
13. The production method for a spunbonded nonwoven fabric according to any one of claims 8 to 12, wherein the conjugate fiber of the step (a) is a polyester fiber.
97