A flat-type lithium ion secondary battery comprising: a positive electrode which comprises a positive electrode active material layer that comprises a lithium transition metal composite oxide of a layered crystal structure containing nickel and a lithium transition metal composite oxide of a spinel crystal structure containing manganese as positive electrode active materials; and an electrolyte which comprises a compound having an S=O bond in a molecule, wherein a content ratio of the lithium transition metal composite oxide of a layered crystal structure in the positive electrode active material layer is in a range of more than 50% by mass with respect to the total mass of the positive electrode active materials, a content ratio of nickel contained in transition metal atoms other than lithium of the lithium transition metal composite oxide of a layered crystal structure is in a range of 35 to 65% by atom in terms of composition ratio with respect to the total amount of the transition metal atoms other than lithium, and a c-axis length of the crystal structure of the lithium transition metal composite oxide of a layered crystal structure is in a range of 14.180 to 14.240 Å, and the compound having an S=O bond in a molecule comprises 1,3-propane sultone, 1,3-propenesultone, or methylene methanedisulfonate, a value of ratio of a battery area to a rated capacity is 5 cm 2 /Ah or more, the battery area representing the maximum value of projected area of the battery including an outer casing of the battery, and the rated capacity is 3 Ah or more, and wherein the rated capacity is determined as follows: an electrolyte solution is injected into a test cell and then left to stand for about 10 hours, and then initial charge and discharge is performed at a temperature of 25°C according to the following procedures, and the discharge capacity by the initial discharge of the following Procedure 2 is regarded as the rated capacity: Procedure 1: Charge is performed for 12 hours at a constant-current/constant-voltage charge of 0.2 C/4.15 V, and rest for 10 minutes is carried out; Procedure 2: After reaching 2.5 V at a constant-current discharge of 0.2 C, rest for 10 minutes is carried out. a positive electrode which comprises a positive electrode active material layer that comprises a lithium transition metal composite oxide of a layered crystal structure containing nickel and a lithium transition metal composite oxide of a spinel crystal structure containing manganese as positive electrode active materials; and an electrolyte which comprises a compound having an S=O bond in a molecule, wherein a content ratio of the lithium transition metal composite oxide of a layered crystal structure in the positive electrode active material layer is in a range of more than 50% by mass with respect to the total mass of the positive electrode active materials, a content ratio of nickel contained in transition metal atoms other than lithium of the lithium transition metal composite oxide of a layered crystal structure is in a range of 35 to 65% by atom in terms of composition ratio with respect to the total amount of the transition metal atoms other than lithium, and a c-axis length of the crystal structure of the lithium transition metal composite oxide of a layered crystal structure is in a range of 14.180 to 14.240 Å, and the compound having an S=O bond in a molecule comprises 1,3-propane sultone, 1,3-propenesultone, or methylene methanedisulfonate, a value of ratio of a battery area to a rated capacity is 5 cm 2 /Ah or more, the battery area representing the maximum value of projected area of the battery including an outer casing of the battery, and the rated capacity is 3 Ah or more, and wherein the rated capacity is determined as follows: an electrolyte solution is injected into a test cell and then left to stand for about 10 hours, and then initial charge and discharge is performed at a temperature of 25°C according to the following procedures, and the discharge capacity by the initial discharge of the following Procedure 2 is regarded as the rated capacity: Procedure 1: Charge is performed for 12 hours at a constant-current/constant-voltage charge of 0.2 C/4.15 V, and rest for 10 minutes is carried out; Procedure 2: After reaching 2.5 V at a constant-current discharge of 0.2 C, rest for 10 minutes is carried out. Procedure 1: Charge is performed for 12 hours at a constant-current/constant-voltage charge of 0.2 C/4.15 V, and rest for 10 minutes is carried out; Procedure 2: After reaching 2.5 V at a constant-current discharge of 0.2 C, rest for 10 minutes is carried out.