A fuel oil flow improver comprising the ester compound and the copolymer, which is the fuel oil flow improver in which the mass ratio [ /] of the ester compound and the copolymer is from 30/70 to 70/30, wherein: - the ester compound is represented by formula: wherein • R 1 is a linear saturated alkyl group containing 17 to 23 carbon atoms; • represents an oxyethylene group; • X, Y, and Z each represent an integer of 1 or higher; • the average number of additional moles (n) of the oxyethylene group is n = (X + Y + Z)/3, and satisfies 1 ≤ n ≤ 3.] - the copolymer: • is of a weight-average molecular weight of 5,000 to 50,000; • is obtained by polymerizing to at a molar fraction of / / = 0.4 to 0.8/0.1 to 0.3/0.1 to 0.3; whith b1 is represented by formula: wherein R 2 represents a linear saturated alkyl group containing 10 to 18 carbon atoms b2 is represented by formula: wherein R 3 represents a linear saturated alkyl group containing 8 to 16 carbon atoms b3 is represented by formula: wherein R 4 represents a linear saturated alkyl group containing 10 to 16 carbon atoms • which is the copolymer in which, when the copolymer is measured using a differential scanning calorimeter and cooled from 100°C to -80°C at 10°C/minute, the exothermic peak temperature (Tp) is -40°C ≤ Tp ≤ -15°C. - the ester compound is represented by formula: wherein • R 1 is a linear saturated alkyl group containing 17 to 23 carbon atoms; • represents an oxyethylene group; • X, Y, and Z each represent an integer of 1 or higher; • the average number of additional moles (n) of the oxyethylene group is n = (X + Y + Z)/3, and satisfies 1 ≤ n ≤ 3.] • R 1 is a linear saturated alkyl group containing 17 to 23 carbon atoms; • represents an oxyethylene group; • X, Y, and Z each represent an integer of 1 or higher; • the average number of additional moles (n) of the oxyethylene group is n = (X + Y + Z)/3, and satisfies 1 ≤ n ≤ 3.] - the copolymer: • is of a weight-average molecular weight of 5,000 to 50,000; • is obtained by polymerizing to at a molar fraction of / / = 0.4 to 0.8/0.1 to 0.3/0.1 to 0.3; whith b1 is represented by formula: wherein R 2 represents a linear saturated alkyl group containing 10 to 18 carbon atoms b2 is represented by formula: wherein R 3 represents a linear saturated alkyl group containing 8 to 16 carbon atoms b3 is represented by formula: wherein R 4 represents a linear saturated alkyl group containing 10 to 16 carbon atoms • which is the copolymer in which, when the copolymer is measured using a differential scanning calorimeter and cooled from 100°C to -80°C at 10°C/minute, the exothermic peak temperature (Tp) is -40°C ≤ Tp ≤ -15°C. • is of a weight-average molecular weight of 5,000 to 50,000; • is obtained by polymerizing to at a molar fraction of / / = 0.4 to 0.8/0.1 to 0.3/0.1 to 0.3; whith b1 is represented by formula: wherein R 2 represents a linear saturated alkyl group containing 10 to 18 carbon atoms b2 is represented by formula: wherein R 3 represents a linear saturated alkyl group containing 8 to 16 carbon atoms b3 is represented by formula: wherein R 4 represents a linear saturated alkyl group containing 10 to 16 carbon atoms • which is the copolymer in which, when the copolymer is measured using a differential scanning calorimeter and cooled from 100°C to -80°C at 10°C/minute, the exothermic peak temperature (Tp) is -40°C ≤ Tp ≤ -15°C.