An object of the present invention is to develop a probe for measuring in real time the kinetics of CREB or actin closely related to brain functions such as memory formation in live animals. The probe used in the present invention is a probe comprising luciferase split into N-terminal and C-terminal fragments, wherein the probe is selected from any one or more of: (1) a probe comprising the KID domain of cyclic AMP response element-binding protein (CREB), the KIX domain of CREB-binding protein (CBP), the N-terminal fragment of luciferase (LucN), and the C-terminal fragment of luciferase (LucC) in one molecule (2) (a) a probe consisting of two molecules, one of which comprises LucN and the KID domain and the other of which comprises LucC and the KIX domain, or (b) a probe consisting of two molecules, one of which comprises LucN and the KIX domain and the other of which comprises LucC and the KID domain and (3) a probe consisting of two molecules, one of which comprises actin and LucN and the other of which comprises actin and LucC.
