NOVELTY - Secondary battery comprises: a positive electrode (3) comprising at least one of a manganese composite oxide having a spinel structure or a metal-phosphorus oxide having an olivine structure; a negative electrode (4) containing a metal compound having a lithium ion absorption potential of greater than or equal to 0.2V (versus lithium/lithium ion); a separator (5) provided between the positive electrode and negative electrode, which comprises cellulose fibers and pores having a specific surface area of 5-15 m2/g; and a non-aqueous electrolyte.    USE - The secondary battery is useful in a battery pack, a car (all claimed) and in an electric train.    ADVANTAGE - The secondary battery: utilizes separator which has high-temperature durability, and at the same time, is superior in output performance at temperatures from room temperatures to high temperatures and low-temperature output performance, because the separator containing cellulose fibers has high heat resistance and thus suppresses thermal shrinkage at a high temperature; and is available at a smaller-size, lighter-weight and with longer life than the lead-acid battery can be attained.    DETAILED DESCRIPTION - Secondary battery comprises: a positive electrode (3) comprising at least one of a manganese composite oxide having a spinel structure or a metal-phosphorus oxide having an olivine structure; a negative electrode (4) containing a metal compound having a lithium ion absorption potential of greater than or equal to 0.2V (versus lithium/lithium ion); a separator (5) provided between the positive electrode and negative electrode, which comprises cellulose fibers and pores having a specific surface area of 5-15 m2/g, and has a porosity of 55-80% and a pore diameter distribution having a first peak in a pore diameter of 0.2 mu m (inclusive) to 2 mu m (exclusive) and a second peak in a pore diameter of 2-30 mu m; and a non-aqueous electrolyte. An INDEPENDENT CLAIM is also included for a battery pack comprising the secondary battery.    DESCRIPTION OF DRAWING(S) - The figure shows a partially broken sectional view of a rectangular type secondary battery.    Positive electrode (3)    Negative electrode (4)    Separator (5)    Metal seal plate (10)    Insulation member (11)