We Claim:
1. A polyester composition comprising polyester as a main component, wherein:
said polyester composition contains a phosphoric acid and an alkali metal phosphate,
a phosphorus element content P (mol/t) is 1.8 mol/t or more and 5.0 mol/t or less relative to the whole of said polyester composition,
said polyester composition contains at least one kind of metal element of Mn or Ca,
any divalent metal element content other than Mn and Ca is 5 ppm or less relative to the whole of said polyester composition, and
when an alkali metal element content relative to the whole of said polyester composition is denoted by M1 (mol/t), and a total amount of Mn element content and Ca element content relative to the whole of said polyester composition is denoted by M2 (mol/t), a metal element content M (mol/t) in said polyester composition which is calculated by expression (i) below and said phosphorus element content P (mol/t) satisfy expression (ii):
M=M1/2+M2 (i)
1.1≦M/P≦3.0 (ii).
2. The polyester composition according to claim 1, wherein said polyester which is the main component of said polyester composition contains a constituent ingredient having at least three carboxylic acid groups and/or hydroxyl groups as a copolymerization ingredient, and an amount of said constituent ingredient is 0.025 mol % or more and 1.5 mol % or less relative to the whole of constituent ingredients in said polyester.
3. A polyester film having a layer (layer P) containing a polyester composition comprising polyester as a main component, wherein:
said polyester is biaxially oriented,
said polyester composition contains a phosphoric acid and an alkali metal phosphate,
a phosphorus element content P (mol/t) in said polyester composition is 1.8 mol/t or more and 5.0 mol/t or less relative to the whole of said polyester composition,

said polyester composition contains at least one kind of metal element of Mn or Ca, any divalent metal element content other than Mn and Ca is 5 ppm or less relative to the whole of said polyester composition, and
when an alkali metal element content relative to the whole of said polyester composition is denoted by M1 (mol/t), and a total amount of Mn element content and Ca element content relative to the whole of said polyester composition is denoted by M2 (mol/t), a metal element content M (mol/t) in said polyester composition which is calculated by expression (i) below and said phosphorus element content P (mol/t) satisfy expression (ii):
M=M1/2+M2 (i)
1.1≦M/P≦3.0 (ii) .
4. The film according to claim 3, wherein said layer P is layer P′ containing a coloring pigment, and, a content amount of white pigment as said coloring pigment is 1 wt % or more and 20 wt % or less relative to the whole of said layer P′, or a content amount of black pigment as said coloring pigment is 0.5 wt % or more and 5 wt % or less relative to the whole of said layer P′.
5. A solar cell back sheet comprising a polyester composition comprising polyester as a main component, wherein:
said polyester composition contains a phosphoric acid and an alkali metal phosphate,
a phosphorus element content P (mol/t) in said polyester composition is 1.8 mol/t or more and 5.0 mol/t or less relative to the whole of said polyester composition,
said polyester composition contains at least one kind of metal element of Mn or Ca,
any divalent metal element content other than Mn and Ca is 5 ppm or less relative to the whole of said polyester composition, and
when an alkali metal element content relative to the whole of said polyester composition is represented by M1 (mol/t) and a total amount of Mn element content and Ca element content relative to the whole of said polyester composition is represented by M2 (mol/t), a metal element content M (mol/t) in said polyester composition which is

calculated by expression (i) below and said phosphorus element content P (mol/t) satisfy
expression (ii):
M=M1/2+M2 (i)
1.1≦M/P≦3.0 (ii).
6. A process for producing the polyester composition according to claim 1, comprising:
an ester exchange process in which an ester exchange reaction is performed by
using a dicarboxylic acid dimethyl ester and a dialcohol in a presence of manganese acetate
and/or calcium acetate as a catalyst, and
a polymerization process in which a polymerization is performed after an alkali
metal phosphate of 0.5 mol/t or more and 3.0 mol/t or less is added to a composition
obtained by said ester exchange process and a phosphorus acid of 0.4 times or more and
1.5 times or less in terms of the number of moles relative to said alkali metal phosphate is
added.