An instruction processing apparatus includes a thread execution processing section executing threads each including plural instructions, a register file including a register window having plural registers, a current window pointer indicating a position of the register where the register window is possible to be inputted and outputted, a current register reading data held by the register window designated by the current window pointer to hold the data and a replacement buffer holding data transferred from the register file to the current register, a first transfer path transferring data in a register file to one of the replacement buffer, a second data transfer transferring data in a replacement buffer to one of the current registers, a calculation section executing a switching instruction of the register window, and a control section controlling, if the calculation section executes the switching instruction, the first data transfer path and the second data transfer path.