1. A method for computer-converting a loop comprising statements to be iteratively executed included in a source program written in a high-level language, to an object program including vector instructions to be executed by a vector processor, comprising: (a) a first step of detecting a set of first to third statements said first statement indicating execution of first data processing to refer to at least one element of array data comprising an ordered set of elements each having an element number; said second statement being executed after said first statement and indicating whether or not branching is to be done, in accordance with a value of a certain condition expression included in the second statement; said third statement being executed in a predetermined one of two cases, when branching is indicated by said second statement and when not, and indicating execution of second data processing to define a value of a larger-numbered element of the array data than the element number of the at least one element of the array data referred to in a same loop iteration by said first statement; (b) a second step of generating, in response to the detection by the first step of the set of first to third statements, a set of vector instructions for executing first to third vector processing; said first vector processing producing vector data comprised of vector elements, wherein an i-th vector element thereof has a value which the condition expression included in said second statement takes at an i-th iteration of the loop; said second vector processing either executing or not executing said second data processing designated by said third statement for at least one element of the array data depending on a value of an element of the vector data having a same element number as said at least one element of the array data; said third vector processing executing said first data processing to each element of the array data after said second vector processing.