By using a bioluminescence, a method for detection of a monovalent cation is provided and the method is a high sensitive and simple method. According to the present invention, an amino acid sequence of symplectin and a base sequence encoding the protein are provided. Symplectin is a photoprotein derived from Symplectoteuthis oualaniensis (okinawan squid). By using the photoprotein of the present invention, a monovalant cation can be detected by luminescence in the presence of a chromophore.