1. A method for monitoring differentiation into cardiac muscle cells comprising: introducing, into cells, a reporter gene of luminescent protein configured to vary in luminescence intensity according to an expression of a myocardial differentiation marker gene, wherein the reporter gene is a cTnT gene expression specific luminescent vector, wherein the cells are stem cells or cardiac progenitor cells; keeping, in an alive state, the cells into which the reporter gene of luminescent protein were 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, the step of acquiring the sequential images comprising acquiring a bright-field image of the cells; associating biological information obtained from the sequential images with biological information obtained from the still image; conducting a motion analysis based on the bright-field image, wherein the motion analysis is conducted by use of an image correlation method; analyzing beating of the cells by the motion analysis; analyzing expression of the myocardial differentiation marker gene based on the luminescence image; and assessing a relationship between the beating and the expression.