In the information security, cryptography is a main topic. This study takes up elliptic curve cryptography which is classified public key cryptography. In elliptic curve cryptosystems, a scalar multiplication is the dominant computation. For speeding up elliptic curve cryptosystems, it is indispensable to improve a scalar multiplication. In computing scalar multiplication, it is important to consider how many registar can use. So this study aims at improvement of scalar multiplication in various situations.