A polyoxalate copolymer including an acid-constituting unit derived from an oxalic acid and a diol-constituting unit derived from a diol component, the diol-constituting unit being derived from a combination of an ethylene glycol and other diol, wherein the other diol is a 1,3-propylene glycol or a 1,4-butanediol, and wherein the polyoxalate copolymer has: - a temperature differential between a 5% weight loss temperature as measured by the TG-DTA and a melting point thereof in a range of 55 to 80° C, wherein the 5% weight loss temperature and melting point are as measured on the TG/DTA 7220 apparatus manufactured by Hitachi High-Tech Science Co. when preparing amounts of samples of 5 to 10 mg, when measuring conditions are nitrogen atmosphere, elevating the temperature at a rate of 10° C /min. and measuring over a range of 40° C to 300° C, and wherein the melting point is found based on the peak top and the 5% weight loss temperature is the temperature of when the weight of the sample has decreased by 5% with respect to the initial weight; and - a quantity of heat of fusion ( Δ H) is in a range of less than 60 J/g, wherein the heat of fusion ( Δ H) is the sum of the areas of the following peaks when measured by differential scanning calorimetry on the DSC6220 apparatus manufactured by Seiko Instruments Inc: (i) when describing a curve of a temperature rise of the first time by elevating the temperature from -100° C at a rate of 20° C /min., observing an endothermic peak therefrom; and (ii) next, further elevating the temperature therefrom by 20° C, maintaining the temperature for 10 minutes, thereafter lowering the temperature down to -100° C at a rate of 20° C /min, then elevating the temperature again at the same rate as above to describe a curve of temperature rise of the second time, and observing an endothermic peak therefrom. - a temperature differential between a 5% weight loss temperature as measured by the TG-DTA and a melting point thereof in a range of 55 to 80° C, wherein the 5% weight loss temperature and melting point are as measured on the TG/DTA 7220 apparatus manufactured by Hitachi High-Tech Science Co. when preparing amounts of samples of 5 to 10 mg, when measuring conditions are nitrogen atmosphere, elevating the temperature at a rate of 10° C /min. and measuring over a range of 40° C to 300° C, and wherein the melting point is found based on the peak top and the 5% weight loss temperature is the temperature of when the weight of the sample has decreased by 5% with respect to the initial weight; and - a quantity of heat of fusion ( Δ H) is in a range of less than 60 J/g, wherein the heat of fusion ( Δ H) is the sum of the areas of the following peaks when measured by differential scanning calorimetry on the DSC6220 apparatus manufactured by Seiko Instruments Inc: (i) when describing a curve of a temperature rise of the first time by elevating the temperature from -100° C at a rate of 20° C /min., observing an endothermic peak therefrom; and (ii) next, further elevating the temperature therefrom by 20° C, maintaining the temperature for 10 minutes, thereafter lowering the temperature down to -100° C at a rate of 20° C /min, then elevating the temperature again at the same rate as above to describe a curve of temperature rise of the second time, and observing an endothermic peak therefrom.