PURPOSE: To change a reference ratio by changing the level of a reference voltage or a reference current by an external signal and fixing the level independently of the external signal thereafter.

CONSTITUTION: When an external terminal T1 is set to the low level from the outside, an N-channel transistor TR NT1 is turned off, and a current does not flow. Therefore, a node TO1 goes to the low level, and a TR NT10 is turned off. Then, the reference ratio is 1.5:1. Meanwhile, when an external terminal T2 is set to the low level from the outside a node T02 is set to the high level, and a TR NT8 is turned on, and therefore, the ratio is 3:1. Thus, the ratio is changed to 4 kinds by levels of external terminals T1 and T2 to perform the test. The speed, the power margin, etc., are measured by these 4 kinds of ratio to determine the optimum reference ratio. If 1.5:1 is the optimum ratio, the ratio is fixed even when a voltage in the low level is given to an external terminal the inverse of W and a voltage is applied to an external terminal VW1 with TRs NT2 and NT1 off and a TR NT3 on to burn out a fuse F, and the optimum ratio is given.