To provide a cross-copolymer excellent in terms of flexibility, tensile property, transparency, and blocking resistance and a medical single-layer tube. A cross-copolymer which comprises: 75-95 mass% main chains of an aromatic vinyl/olefin copolymer made up of 8.99-15.99 mol% units of an aromatic vinyl monomer, 84-91 mol% units of an olefin monomer, and 0.01-0.5 mol% units of an aromatic polyene monomer； and 5-25 mass% cross-chains of a polymer made up of units of an aromatic vinyl monomer. When examined in a nitrogen gas stream of 30 mL/min by a DCS method in which the cross-copolymer is cooled to -50 DEG C, subsequently heated to 180 DEG C at a heating rate of 10 DEG C/min, cooled again to -50 DEG C, and heated to 180 DEG C at a heating rate of 10 DEG C/min, then the cross-copolymer has a fusion peak top temperature of 60-80 DEG C and a heat of fusion of 45-75 J/g, the heat of fusion being calculated from the area surrounded by a straight line drawn between the -20 DEG C and 130 DEG C points of the DSC curve and by the DSC curve between -20 DEG C and 130 DEG C.
