1. A method for monitoring differentiation into cardiac muscle cells comprising: keeping, in an alive state, cells into which a reporter gene of luminescent protein configured to vary in luminescence intensity according to an expression of myocardial differentiation marker gene is introduced; acquiring a luminescence image as a still image by imaging light emitted from the cells in a light shielding state; acquiring sequential images with illuminating the cells; and associating biological information obtained from the sequential images with biological information obtained from the still image.